What I'd do is get another 2.5 gallon tank. That's pretty small for a couple of bettas. Just cuz they CAN live in small space, doesn't mean they're happy! How would you like to live in one room in a house your entire life? The filter would probably be a bit strong for the little guys in such a small space. As for bumping against the divider, i don't know... I only have one betta in a 5 gallon minibow.... he's a sweetie!