I've read some info about this and it DOES seem possible, but the tank has to be set up for it. Each end has to feel like a territory, and like Oldfishlady has in hers, the info I read said the inclusion of females was necessary. If there's no females, they'll kill each other thinking that there's now 2 guys, and IF a girl shows up here, that guy's competition.
So two nice territories, heavily planted, and plenty of food with females, I think it's possible. But if you're going to make a huge tank like that, and it looks like two seperate tanks... do you really want just 2 bettas in it? You can get other nice colourful fish like cichlids in a 75.