You got lucky the first time. Unfortunatly, female bettas are not fine together. female bettas can be quite agressive to other bettas.
When you attempt to keep female bettas together it is call a beta sorority. You need to have at least a ten gallon tank and at least 5 female bettas. And a lot of silk or live plants. in a larger tank, you can keep more then 5 bettas, for possibly better luck. They will form a pecking order. You need to ad all the females the same day. The beta sorority doe not always work!, If you end up with one very aggressive female "like i did" you need to remove her. but then, If you only started with 5 betas, you wont have enough fish to form a pecking order. Them the remaining fish are likely to kill each other. "Like Mine did."