No just one per tank, and I just checked they are 65 watt. Because it spreads the heat out more the heater itself actually doesn't get as hot as a che. I haven't had any problems with the top of my exoterra.
I have the RHP on top of the screen lid and then cover pretty much all the rest of the screen with plastic wrap to hold humidity and a cloth on top of that to hold a little more heat. It's working great for both my BPs but of course everyone is different.
Its on top of the enclosure? I just watched a video on youtube and it looked like they installed inside the tank.
I have a 40 gal with a uth and che, my che keeps the (air) temperature the right temp as long as my room temperature is not under 69 degrees, i have the uth on the hot side as well which gets it to the right temperature as well i keep a towel on top of part of the screen lid which slightly helps with containing te heat hope this helped