If you don't want to get more bettas, I think the easiest and safest thing to do would be to just put a tank divider in so each girl gets 5 gallons. That's plenty of space, and bettas don't need company, plus they'll get entertainment from seeing each other through the divider. But yeah, you'll want a heater, bettas like temps of 78-80 degrees. You'll also want a filter, but make sure the current isn't too high (although females can handle a little more current than males). Bettas also like to have little caves and stuff to explore. Also, as long as you keep the water clean, I've found that bettas are tough fish that give me little trouble and have nice personalities, so you should come to like them.