yesterday I took the divider out of my 5 gallon and moved one of the bettas into a 2 gallon jar..
this was for two reasons, one is that in case of illness it could spread to the other, and the second reason is that one of the bettas I had in there before jumped the divider..
so I would have to say that I won't have a divided tank anymore, and I would make sure that your divider is high enough and tight enough against the tank walls that one betta can't get to the other.. they might look like they are ignoring each other but its only a game