Just to clarify, the tank is divided so that each betta has his side, and then in the middle, between the mesh dividers, is the heater and filter. I can't see on the other side, and they haven't noticed each other.
Anyway, his color has improved since I came home from work, but now I notice sometimes he's twitching :\ I checked him with a flashlight, and he doesn't have anything that looks like velvet or ich.
However, just now I caught him "flapping" his gills very rapidly (one gill is still out more than the other), and again all these bubbles came out. He seemed very distressed by it, and tiny bubbles continued to come out even though he'd calmed down.