For some reason PetCo sent me an email asking to answer this question. The answer is no. It does not come with a humidity gauge. It came with a temperature gauge that does not work very well, so I would get a good gauge that does both if you want to get this kit. Or get two different gauges for temp and humidity. 

None

Yes it's a great starter kit for a juvenile dragon I would start with the 20 Gallon kit then once fully grown upgrade to 40 gallon

I believe it wouldn't but it depends on whether their adults or babies, if it's a setup for babies than yes, if it's for adults then I think it's a 40 gallon minimum. Make sure you quarantine them for 2 weeks, if they have parasites than it could spread to other reptiles (if you have any.) Also if you get 2 leopard gecko's from 2 different places than only 1 has to be quarantined for 2 weeks before their introduced to each other. But to short answer, If it's 2 adults than no, but if it's 2 baby's than you can probably get away with it. just make sure you upgrade their tank when they turn about 1 year old (that's when they become adults) and make sure you do extensive research on co-habing Leo's

Yes, you probably could get away with it, just make sure they get along well and that they both eat food because sometimes they'll lay on top of eachother as a sign of dominance and they'll eat all the food on the other one. Do extensive research on it first

Bearded Dragons don't require under tank heaters. They need basking lights and Uv lighting.

Not recommended for water but if land turtles yes.