I voted two but not because I use one to heat the cool side. I have a ceramic heat emitter in the middle (but more on the warm side) of my enclosure with a UTH under his warm hide. The ceramic heat emitter keeps my 20 long at 80-82 constantly. The UTH makes it about 92 in his warm hide.

I really like the ceramic heat emitter. I've had the same bulb for literally years. They worked great for the baby tortoises I had prior to acquiring my BP.