I completely agree with what KMG said in his post above.
For your question though, yes 2 UTH's at most is going to be 50ish watts, and likely to be much less. Most UTH's have their wattage printed on them somewhere, and the packaging should say so as well. Just add each UTH's wattage and subtract the number you get from the thermostats capacity (in this case 500 watts) I advice to keep 10% of the thermostats capacity unused to be safe (so a 50 watts safety zone for a 500 watt thermostat)
In a similar price range you can get a hydrofarm thermostat. While thee aren't great either they are better than the zoomed unit.
http://www.amazon.com/Hydrofarm-MTPR.../dp/B000NZZG3S
This is a nice mid-range thermostat:
http://www.reptilebasics.com/thermos...stat-prewired/
And as KMG posted the herpstat line, these are the best you can get right now. I would recommend the herpstat intro +, Herpstat 1, Herpstat 2, or Herpstat 4 depending on how many cages you want to heat:
http://spyderrobotics.com/home/products.html