You can keep more than one in a terrarium, but these are large tree frogs that need a lot of space. I think you could keep a total of 2 adults in that terrarium size. That's just my opinion.

Generally it's a good idea to keep the new frog on its own in a temporary container for a few weeks, just to make sure it's eating well and doesn't display any health problems. Then you can go ahead and introduce the new one to the old one, just be sure there isn't a huge size difference.