I've found dual UTHs dont really bump ambient temps much in larger setups. They worked well for small tubs, but even my 17L RUB barely got a 4 degree bump with an 11" secondary flexwatt and a towel insulating the top and sides.
For a larger enclosure, a RHP has worked much better.
Pardon my ignorance but is a RHP essentially a UTH attached to the rear back of the cage? And if so, wouldn't a UTH conceptually work better w/ ambient temps since heat rises? Again, I'm a beginner and just trying to gain information.