Bettas are agressive no matter what! Never keep two together! Females can only be kept together in groups of four or larger and ten gallons is the minimum.
Bettas were first bred for fighting, and when it got bred for beauty, it never lost it's aggression. And mirrors are perfect to get you bettas to show their true colors! I had a dull female, but she turned into a black female with brilliant red fins when I "exercised" her. :)