You're better off putting the second UTH underneath the enclosure, with its own thermostat set in the low 80's and a hide over top.
Another alternative is a deep heat projector (DHP). These are fairly new to the market and emit IR radiation. The waves penetrate plexiglass fairly well if the glass is clean and unscratched, regular glass not so much. Once the IR wave hits a solid object like cage décor or your snake, the energy turns into heat. I picked up a couple to use with my carpet python so he can have a warm-side perch as well as hide, and I'm very pleased with the DHP so far.