I would keep the RHP for ambient temperature and the UTH for belly heat.

As others have said, the sensor for the thermostat goes between the UTH and the glass tank bottom. Using your temp gun, you should measure the temperature directly on the glass tank bottom without any substrate.

Don't worry if the surface temperature above your UTH is lower than 90 degrees after you add substrate. Your BP WILL move the substrate out of the way if he wants more heat.

And you can save some money by not putting down too much substrate. By the time your BP weighs a few hundred grams he'll be pushing around anything and everything that's in his way.