I have both a rhp and a uth in each of my large cages. I run it with a Herpstat4 and all is well. As long as the power draw of each device added together does not overload the tstat it should be fine.
You could also use a thinner piece for the floor so the uth can be mounted under each cage.
Also another thing to note is the rhp will cause heat on top of it so it heats the cage floor above it. I had to turn the rhp's down because they were creating a hotspot in the tank above them that was higher than I wanted for my BRB and Blood. Since your building yours you may want to put a thicker foot on each cage to separate them a bit more than mine are. Mine have about 1/8 of an inch.