They are unlikely to attack each other as RTBS usually only attack fish of a similar shape / colouration to themselves or fish that are always in their territory, which tends to be the lower levels of your tank. However, As mentioned I also feel that bettas are totally unsuitable community fish for any tank. As:
1. The can be aggressive to other colourful long finned sp.
2. Their poor swimming ability means they really need a very slow trickle from the filter which suits very few other fish.
3. Poor swimming means they are also crap at competing for food.
4. They are rubbish at escaping, being nipped by smaller faster fish.
Kept in a heavily planted 5 - 10 gal tank on their own and theyre fine though.