I have 2 propane gas water heaters. Do I need a pump on each? And would I expect to see an increase in my gas bill to keep the water hot all the time?

Yes, you will need to install a pump on each water heater. If the water heaters run on separate plumping loops, then you will need to install a sensor valve at the faucet furthers from the heater on each loop. As far as an increase in your gas bill, that really depends on how often you run the pumps. The pump has a built-in timer, so if you are only running the pump at certain times a day when you usually need hot water, you probably won't notice much difference. If you run the pump all the time, you probably will.