Yeah, I totally agree with Bogertophis here. Two CHE's is too much and will dry out the tank.
If the room is 75F, you can put a small CHE in the middle on a low setting to keep the middle of the tank 80-82F or so and the cool side about 78F, is my guess. Then the UTH on the warm side at 87-89F should do the trick for digestion.
Just remember two things.
1. heat sources will reduce humidity. The degree depends on other factors including what heat source, source of humidity, etc. but heat causes humidity to go down.
2. Too much heat is bad too. You can overheat a reptile easier than you can cool one down too much, in most instances. Too much heat (even 95F +) can hurt a BP (neurological issues/damage) and even kill them.
I would keep it simple.
Cover the top of the tank with tin foil and cut out a spot for the CHE in the middle part of the tank. This will keep heat and humidity in (better). Get the UTH to proper temps and you should be good.
Even if the room was 70-72F, a properly controlled CHE and UTH should do the trick in a 20L.
Too many heat sources mean too many adjustments, etc. Relatively speaking, that's a small tank, so the less need to fiddle the better.
Good luck and keep us posted.
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