Seguin drew Boston even at 1 with his second preseason goal 4:17 into the third off a rebound in front of Carey Price. Clark, who was invited to camp, put the Bruins ahead with his first goal at 13:15.
"Knock on wood, the power play seems to be clicking right now," said Seguin, who was drafted second overall by Boston in 2010.
After playing center in Sunday's 7-3 win over Montreal in Halifax, Nova Scotia, Seguin played right wing in the Bruins' second win over Montreal in two nights.
"It's good seeing how everyone plays," Seguin said. "It's a long season. There are going to be lots of injuries, so it's good to see what kind of chemistry can be there."
Tuukka Rask stopped 31 shots for the reigning Stanley Cup champion Bruins (3-0-1).
"I like the fact that we have a habit to let the goalies play a full 60 [minutes]," Rask said. "I think it lets you get in a rhythm and get back to your game vision. I've had no problem with that. Obviously, the first two games might be a little tougher on you conditioning-wise when you're not used to it, but I like it."
Boston beat Montreal in the first round of the postseason last spring, one of the Bruins' record three seven-game playoff series wins on the way to their first championship since 1972.
Michael Cammalleri gave the Canadiens a 1-0 lead with his third goal 15:33 into the second. Cammalleri, who didn't play Sunday, had a pair of power-play goals in the Canadiens' 3-2 loss to Ottawa on Saturday.
Price went the distance and made 30 saves for Montreal (1-5), which signed defenseman Chris Campoli to a one-year, $1.75 million contract before the game.
"At this point in time we're trying to get ready for the season," Canadiens captain Brian Gionta said. "There's two games left, it's time to start getting into that season form and getting your timing back. I thought the intensity was a little better tonight."
After making 10 cuts earlier in the day, the Canadiens reduced their training camp roster to 40 by sending out five of the players who appeared in the game.
Forwards Alain Berger, Philip DeSimone, Brock Trotter and Philippe Lefebvre were assigned to Hamilton of the AHL. Defenseman Jarred Tinordi, Montreal's first-round pick in 2010 who played his third preseason game, was sent back to London of the OHL.
Fellow prospects Olivier Archambault, Nathan Beaulieu and Morgan Ellis were also returned to their junior teams earlier in the day. Alexander Avtsin, Andrew Conboy and Dany Masse, defensemen Mark Mitera and Joe Stejskal and goalies Robert Mayer and Peter Dekmas were sent to the Bulldogs' minor league camp.