Hey, and nice to meet you 
Anyway, DEFINITELY change the substrate. They sell at most pet stores ground up coconut husk, and it's sold under names like Bed-A-Beast, Eco Earth, and others. They come in a brick which you stick in water and out comes something quite like dirt... but it's pretty safe.
You might need a heat lamp if your room gets cold, but as long as it stays around 75-ish, it's fine... you can use a regular light bulb to provide heat in a top corner with those temps. Definitely no heat rock should ever be used, and a heat pad isn't recommended, either.
A ten- or fifteen-gallon is way too small for two American Green treefrogs, and pretty cramped for one. A fifteen- isn't too bad, and it's not like the frog will turn over and die or anything, but you can see a huge difference in activity when you move them to a larger tank, and it will thrive as opposed to just live. A 20-gallon will surely house two, but these animals do not get lonely and do somewhat better alone.
Sexing is kind of difficult without a comparison. Both sexes will have what looks like looseness or loose flaps on the neck, but the males' are much more noticeable. Also, the males usually have yellow coloring on it, or the green carrying on it further than on a female. Also, if it calls, it's a male 
Anyway, good luck with your frog and don't hesitate to ask anymore questions. There are some very knowledgeable people on here who can help you on all but the most extreme problems! Hope to see you around 
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Got hips like Cinderella / Must be having a good shame / Talking sweet about nothing / Cookie I think you're Tame 