1. Another might not be needed but I have seen the change in my frogs activity level when they were alone and when I added a friend. They are always together. Bottom line is as long as its the same species it cannot hurt.

2. Some tree frogs to feel safe will climb to the highest point that they can get to, they do not like things coming at them from above. That is why with tree frogs a good rule of thumb when choosing an enclosure is Height over width and length.

3. As far as the carpet goes, I have never used it personally so I cannot answer how well it holds moisture. I did however hear from some people who have used it say that it has to be washed everyday to every other or it becomes a breeding ground for bacteria.

Hope that info helped.