A ten gallon sounds a bit to small for one leopard frog. You should bump it up to at least a 20 gallon long, but if you want to add another leopard frog I'de get the biggest aquarium you can get. Bigger is always better.
Frogs do not need to have "friends" to keep them happy. If you really want to house two of them together, be aware that they may try to eat each other, and if one frog is larger than the other, there is a good chance you may wake up one morning and find only one frog in the enclosure. Make sure they are from the same locality too, a frog from up north and a frog from down south will have different temps they like to be kept at.
Good luck
Spoony