:leopard gecko as well. It's really not good idea to mix those species in a 75 gallon tank. The tank seems rather large, but in actuality, it is quite small when putting predator and prey together. Collareds are notorious for eating other lizards, and ofen if wildcaught, will readily consume lizards before accepting crickets and mealworms.
:I would set up a desert viv, with the same type of specs you mentioned about, but I would have a basking spot with a temp of 105. They are an awesome lizard to enjoy. I cant stess the separation enough. They may be fine for a while.