I wouldn't divide the 2.5 for 2 bettas. Yes, some people argue bettas are fine in 1g tanks but you'll soon find that a 2.5 can be quite cramped with 2 bettas.
You can add some amano or ghost shrimp (they eat algae and are big enough to defend themselves against a curious betta). Red cherry shrimp are a lot smaller and are shy. The thing is though, ghosts were bred as feeder food and are less hardy than other dwarf shrimp species. Amano are your best bet.
A nerite snail won't reproduce in your 2.5 but they NEED real algae to live. They won't eat algae wafers since a lot of them are still wild caught. They also need extra special care in acclimation.
Like Lizbeth suggested, put another tank next to you bettas, the two of them will have their own space and still be able to visually communicate with each other.