Chinese Water Dragon Mate

Okay, so I have a two year old Chinese Water Dragon (male) and I was wondering at what point could I get him a mate and how old would they have to be? Right now I have a 55 gallon tank for him in but once they get a little bigger, I would sell that one and buy the next size they would need. That part wouldn't be a problem but I would also like to know how many more gallons I should add for them to be insured both to be living together in for the rest of their lives. Oh, and I have a 20 gallon tank that the other CWD would be quarentined in. :)

Thank you! :D