You can certainly divide a 10 gallon for 2 bettas. I always used the craft canvas/report spine method and it worked pretty well. Mine just stayed in place due to pressure, but if you're worried about escapees you can always silicone it in place since you haven't set up the aquarium yet. What I did to keep even water flow/ heat distribution was to position the filter intake tune on one side of the divider and let the outflow lie on the other side (for a HOB filter), which requires you to cut a little notch out of the top of the divider where the outflow crosses it. I had a mini canister filter on one 10 gallon which was even easier as you just position the intake tube on one side and the outflow tube on the other. In either case I positioned the heater right next to the intake tube so that the filter would distribute the heat.
My dividers worked pretty well. My only complaint, and why I probably won't do it again, is that they were a great place for algae to grow and were pretty hard to clean without making a giant mess.