As far as painting the back goes, why don't you just use a piece of paper in the appropriate color and attach it to the back of the tank using a piece of tape or similar adhesive? It sounds easier to implement and would be easier to change if you decide to use this tank for something else in the future.
I agree with siva in that I don't think male bettas should be able to see each other constantly. I've read from multiple credible sources that they will kill themselves TRYING to fight with one another. I think you should use a moss wall between the two of them, placing it such that it hides the seam between your two pieces of paper. This way, it would look neat (in my opinion, anyway) and would keep them from being able to see each other. Or as sarasein said, you could just use a couple moss walls to HIDE the divider.