Theory needs justification

My tank for my other child is a 40 gal. Glass enclosure, covered up the screen up top except for ventilation.
3ft long x 18inches deep x 17in high.

If using two 30-40 gallon UTH's, one for hot, one for cold, would i be able to keep ambient temp stable, and humidity as well? I hope someone can answer, or i may be forced to spend more money, oh no. :D

I'm pretty sure the ambient temp wouldnt get up very much, unless you covered the top part way.

not sure why you would need two heat pads...

Only you can find out by trying ambient temps and humidity will depends on various factors that vary from one owner to the other mainly due to their own ambient temp inside their home and humidity inside their home as well.

It should work but will require some tweaking around most likely.

I personally think you're trying to put more to this than is neeeded. Put the UTH in for the hot side. Adjust the temps on that side and see how the rest of the tank goes. Don't buy things that you don't need. Most people are fine with just one UTH.

Good Luck!
Jim Smith