Since a good temp for a leo warm side is around 85, I don't see a problem as long as that UTH is on a t-stat. My adult male is in a 10gal tank with a UTH under the warm side and two timed lamps (day and night since these guys are nocturnal) holding 40wtt bulbs. The day lamp is aimed over the warm side and the night lamp is sort of central/warm side. The room I keep him in is also rather dark in the daytime, so I use the lamps as his time reminder as well as for added ambient heat.

Since you have a 20gal, I would place the UTH under your warm side hide on a t-stat to have the substrate surface at 83-85 by digital thermometer. The lamp can be above his warm side for general light and added warmth. Just make sure the bulb you put in isn't too strong and you should be fine.

Of course, if any of the true experts here say different, then I sit corrected.