2 gallons of water per betta is perfectly fine.

Keep up with water changes so the ammonia doesn't build up. Also, I use fresh water aquarium salt in ALL my tanks (not just in my betta tank) every time I do water changes. I know some people only use the salt when their fish get sick, but I swear by the stuff. My fish have NEVER gotten sick from anything other than old age things since I started using the salt. Since your tanks are small a small carton will last you a long time. If you do get the salt make sure it's FRESH water aquarium salt and for the first few water changes add it slowly until your bettas get used to the amount. My betta, Typhoon, is used to it so I add the whole amount at the beginning. My 5 gallon tank only needs 1 tablespoon. It's GREAT stuff!

Congrats on the girls~