I'm not actually sure on minimum size but I know one person on here who keeps a betta in a 2g
Oh, by the way, that doesn't mean its correct to do so.
I just know they have kept betta's for a long time and have kept it in a 2g. You may want more advice from a more experienced member than me though as I know very little about them.
You can but many of Betta lovers would prefer to see them in bigger tanks. The bigger the better is how many of us look at it but what we all should consider is....can we give them a better home than the cups they are in in the stores? So from that perspective, I'd rather see a betta in a 1 gallon tank than a little cup in the store where the chances are very good that poor Betta will die in that cup. They rarely feed the bettas in the stores because:
A: it makes them grow faster
B: it means they have to do more frequent water changes which is a chore when they have about 60 cups to change.
For me, the answer to your question would be yes. You can do that BUT make sure the divide is not translucent. They would be too close and seeing each other so close would stress them out.
My recommendation....Find a little bigger area, get a 10 gallon tank with two non-transcucent dividers and save three Bettas.
If you have the room, 10 gallon tanks are dirt cheap and they allow for a filter and a heater. I personally don't like using dividers but in the name of saving another Betta I wouldn't let it stop me.