It's just conjecture but I'm going to guess that there's not really a lot of work for twins who are older than five or so (to play the same character). I think the Olsen twins had that locked up in the 90s and everyone got tired of it.
But, think of it... when was the last time you saw a film or TV show that called for identical twins? There were the Mowry sisters in Sister, Sister, The Parent Trap (played by one person in both versions), The Harry Potter Series (minor characters), Orphan Black (one person)... I don't see it often... and with digital capabilities now, it's probably easier to find one actor to play both (or all 6-8) roles than to find actual twins to do it... who are actors... and are equally talented.
Just because they look the same doesn't mean they have the same interests and abilities. Maybe one doesn't want to do comedy because they have no comedic timing... maybe one only likes serious work... it's possible that they'd love to work together if the right project came along. Another explanation could be that their schedules just don't work... if one is on a TV show, they may be unavailable to do a film.
They have apparently worked together in more than an extra role on a 2010 episode of Fringe, though. They play twins Matthew and Joshua Rose on season 3 episode 5, Amber 31422.