Just to update, I ended up buying the largest UTH a local reptile shop had to offer, and put 3/4 of it on the ball python's side and 1/4 on the corns side, and plugged it in to the corn's thermostat. I swapped out most of the bark for aspen shavings, with of course a shed box.
Currently the ambient temp throughout is maintaining at 77F, but the bottom of the vive is ranging from the low 80's on the cool side to the mid-high 80's on the warm side (but that one is still rising slightly).
Interestingly, my BP is staying away from both heating pads :S maybe he prefers a cooler climate...