At first I used a white divider. Hated it! My substrate is black so it just stood out too much. Yesterday I got a black one and fitted it in today. Much better!
The bettas are more interested in playing in the bubbles at the back of the tank then each other. When I changed the divider they ended up together somehow. Flared some at each other but then was like, whatever and swam off. Both of them are brighter and seem happier. One of them, Louis, I've only ever seen as blue, but since going in this tank he's turned part red through his fins too! He is working on growing back some fins that were nipped of by a tetra in my community tank. The other one, Prince had turned a lavender color with some blue and red woven in. Both are so pretty now!
Thank you so much for your help!