I highly recommend this tank from Walmart...
It's under $30, and is a glass tank with the full set-up -- hood, light, filter and filter cartridge, the only thing you'd need to buy is a heater. This is the tank I have my betta Tofu in by himself. And it could be divided, so that each girl got 2.5 gallons to themselves.
In the meantime, how often are you doing water changes on the girl's divided container? You need to do a 100% water change every other day, otherwise it is going to affect their health greatly, because of the ammonia build-up from two fish in such a small enclosure.