"I told them at the start of the year if they're playing well they're both going to play," Ruff said.
Lehtonen and Niemi have equally split the starts through 20 games this season, which wasn't intentional but is a byproduct of how they're playing, Ruff said.
Dallas entered its game Thursday against the Washington Capitals with 30 points, first in the Central Division, arguably the deepest and toughest division in the NHL this season.
Lehtonen, who was the starter in Washington, had eight wins, a 2.38 goals-against average and .921 save percentage in his 10 starts. Niemi had seven wins, a 2.63 GAA and .911 save percentage in his 10.
It helps the goalies that the Stars have the best offense in the NHL, which widens the margin for error and takes away some of the pressure. They were averaging a League-high 3.58 goals per game and were second on the power play at 29.3 percent entering Thursday.
"If you ask me in the next 20 games if they're going to play 10 each, I can't give you the answer on that, but they've both played well and we don't want one to sit too much longer than the other one," Ruff said. "At the same time, they're getting a lot of valuable work with [goalie coach] Jeff [Reese]. If we're still going real well we may run with one guy for three or four."
Niemi started four games in a row once. Lehtonen's season-high so far is three straight starts, which he's done on two occasions.
Ruff, though, pointed out the Stars schedule in the first quarter was conducive to letting one of them play if he gets hot in order to give the other one a long stretch of rest and extra practice time with Reese. Dallas played on back-to-back days once in the first 20 games.
The schedule gets harder from here. The Stars have 11 sets of back-to-back games over their final 60 games.
"One guy could really put a string together and I guess you could put the No. 1 label on him," Ruff said, "but I still look at these next 60 games, when the schedule starts to get heavier, we start playing back-to-back, and we want to be able to use both guys."
There are six other teams that went into the season with plans to have at least two goalies split starts. Here is a breakdown with a verdict on how it's working:
Detroit Red Wings: Petr Mrazek - Jimmy Howard
Mrazek has started 10 games. Howard has started nine. Mrazek has five wins, a 2.16 GAA and a .934 save percentage. Howard has four wins, a 2.29 GAA and a .918 save percentage. He has faced 90 fewer shots in 86 fewer minutes.
Verdict: Working well
St. Louis Blues: Jake Allen - Brian Elliott
Allen is pulling away with the No. 1 job. Elliott rebounded with a strong performance against the Columbus Blue Jackets this week. Allen had a 1.91 GAA and .934 save percentage in 12 starts entering Thursday. Elliott had the other seven starts with a 2.70 GAA and .898 save percentage.
Verdict: Allen making it work
Ottawa Senators: Craig Anderson - Andrew Hammond
Hammond's injuries have allowed Anderson to secure the No. 1 job. His grip on it is soft. Anderson had a 3.00 GAA and .911 save percentage entering his start Thursday against the Columbus Blue Jackets. He allowed four or more goals in five of his first 13 starts. Hammond is currently dealing with a head injury.
Winnipeg Jets: Ondrej Pavelec - Michael Hutchinson
Neither has great numbers, but part of the problem is the Jets are killing a League-high 4.1 penalties per game (five per game in their past six). Pavelec has started 12 games with a 2.81 GAA and .906 save percentage. Hutchinson has started eight games with a 2.82 GAA and .913 save percentage. Their combined even-strength save percentage of .920 is 23rd in the NHL.
Toronto Maple Leafs: James Reimer - Jonathan Bernier
Reimer has run hot of late, but coach Mike Babcock's faith in Bernier appears to be shakier than the goalie's confidence, which isn't exactly stable at this point. Reimer has allowed only 10 goals, including seven at even strength, in going 4-0-2 in his past six starts. But there are questions about his long-term stability as a No. 1 goalie. It's also too early to give up on Bernier.
Calgary Flames: Karri Ramo - Jonas Hiller - Joni Ortio
They're the only team that has had three goalies on its roster all season. It hasn't worked. Ramo leads the trio with 12 starts, a 3.22 GAA and .898 save percentage. He was tied for 25th in save percentage and 29th in GAA among goalies with 10 or more starts. Hiller and Ortio have been worse. The goalies have a combined .884 save percentage and 3.48 GAA.
Verdict: Not working
DISCLAIMER: While the Super 16 is NHL.com's weekly power rankings, the focus will be more on the "power" than the "rankings" when determining the order. It's not always going to look like the League standings. If two teams are close the tiebreaker almost always is this: If the two teams started a seven-game series right now, who would prevail?
All rankings, records and statistics are through the games played Wednesday night. Don't ignore the scattered links to good stories either:
1. Dallas Stars (16-4-0)
Last week: No. 4
What's good: Valeri Nichushkin is starting to become a consistent scoring threat again because he's using his size and speed to his advantage the way he did as a rookie two seasons ago.
What's not so good: Hard to nitpick at a flaw in their game at this point.
Read more on Nichushkin in the Over the Boards blog: http://www.nhl.com/ice/blogpost.htm?id=42974
2. New York Rangers (14-2-2)
Last week: No. 3
What's good: Mats Zuccarello is playing like an all-star, which nobody could have predicted after witnessing his scary and potentially life-altering head injury in the Stanley Cup Playoffs last season.
What's not so good: They're still a product of some luck with a 106.6 SPSV% (shooting percentage + save percentage) entering play Thursday. What happens when that goes down?
3. Montreal Canadiens (14-3-2)
Last week: No. 1
What's good: Carey Price, who has missed nine straight games, is on the mend and should be back in the lineup soon.
What's not so good: The experiment with Alexander Semin is not working out.
4. Los Angeles Kings (12-6-0)
Last week: No. 6
What's good: Forward Marian Gaborik had a goal and an assist in L.A.'s 4-3 win against the Edmonton Oilers on Saturday.
What's not so good: Gaborik's two-point game doubled his point total for the season.
5. Washington Capitals (12-4-1)
Last week: No. 5
What's good: Evgeny Kuznetsov gives everyone constant reminders of how special he is and how much better he can get.
What's not so good: Entering Thursday, the Capitals had given up the first goal in 10 of their first 17 games.
Braden Holtby talks about facing more shots on goal in Kevin Woodley's Unmasked this week: http://www.nhl.com/ice/news.htm?id=788646&navid=nhl:topheads
6. Nashville Predators (11-3-3)
Last week: No. 7
What's good: Pekka Rinne has allowed three goals on 83 shots in his past three starts, and he has the best goals-against average (2.10) and save percentage (.921) of the seven goalies in the Western Conference who have started at least 15 games.
What's not so good: Filip Forsberg has gone 13 straight games without a goal and has two this season. He has seven goals in his past 38 regular-season games.
7. Chicago Blackhawks (11-7-1)
Last week: No. 10
What's good: With 30 points in 19 games, Patrick Kane is on pace for 129 points this season. He's pretty good.
What's not so good: Trevor Daley has not fit in well on the blue line, and his ice time is taking a hit. He has topped 14 minutes twice in the past eight games after going over it 10 times in the first 11 games, including seven times over 17 minutes.
8. St. Louis Blues (12-6-1)
Last week: No. 2
What's good: Paul Statsny (broken right foot) skated with the team Thursday morning, a good sign that his return could be coming soon.
What's not so good: Rookie defenseman Colton Parayko has missed two straight games.
9. Minnesota Wild (10-4-3)
Last week: No. 8
What's good: They play six of their next seven games and 12 of their next 17 games at home, where they are 7-1-0 this season.
What's not so good: They have trailed in the first period in eight of nine road games entering play Thursday.
10. Pittsburgh Penguins (11-7-0)
Last week: No. 9
What's good: They entered their game Thursday with a 9-0-0 record when scoring the first goal and 6-0-0 when leading after the first period.
What's not so good: They were 2-7-0 record when giving up the first goal and 0-5-0 when trailing after the first, calling into question their ability to overcome deficits.
Brian Metzer looks into Kris Letang's value in fantasy hockey: http://www.nhl.com/ice/news.htm?id=788502
11. New York Islanders (10-6-3)
Last week: No. 12
What's good: Kyle Okposo has five points in the past three games and seven points in the past six. He's again producing on a line with John Tavares.
What's not so good: Anders Lee has three goals in 19 games, including one in the past 14. He scored 25 goals in 76 games last season.
Jon Lane wrote about how the Islanders are trying to pick up fans and make some hockey players in Brooklyn: http://www.nhl.com/ice/news.htm?id=788389
12. New Jersey Devils (10-7-1)
Last week: NR
What's good: They're scoring 2.44 goals per game, which doesn't sound like much until you consider they haven't scored that much since the 2011-12 season, when they scored 2.63 per game, made the Stanley Cup Playoffs and went to the Stanley Cup Final.
What's not so good: They're averaging a League-low 25.8 shots on goal per game.
Here's a good feature on Travis Zajac by Mike G. Morreale: http://www.nhl.com/ice/news.htm?id=787774
13. San Jose Sharks (10-8-0)
Last week: NR
What's good: They were 7-3-0 on the road entering their game Thursday at the Philadelphia Flyers.
What's not so good: They're only 3-5-0 at home.
14. Tampa Bay Lightning (8-9-3)
Last week: No. 13
What's good: Goaltending has been their strength as Ben Bishop, Andrei Vasilevskiy and Kristers Gudlevskis had combined for a .923 save percentage (517 of 560) entering play Thursday.
What's not so good: They've struggled to make the goaltending matter because they were scoring 2.30 goals per game and were shooting 8.1 percent (46 goals on 567 shots).
15. Florida Panthers (8-7-3)
Last week: NR
What's good: Defenseman Aaron Ekblad entered Thursday with four points on three goals and one assist in his previous four games after scoring only three points in his first 14 games.
What's not so good: The Panthers were being outshot 216-155 in the second period and were allowing 31.3 shots on goal per game, which was 27th in the League.
16. Winnipeg Jets (9-9-2)
Last week: No. 11
What's good: The hope is they can build on their 4-1 win against the Vancouver Canucks on Wednesday, when they played fast and with good structure.
What's not so good: They lost their previous six games (0-5-1) and fell to sixth in the Central Division, 10 points off the lead, after being third and only two points off the lead two weeks ago.
Out: Vancouver Canucks, Arizona Coyotes, Boston Bruins
Follow Dan Rosen on Twitter at: @drosennhl
Author: Dan Rosen | NHL.com Senior Writer