A new playmate for my African Dwarf Frog?

for dwarf frogs it is suggested 1 gallon per frog ... when i bought mine one was big and one was small but with regular feeding they are both around the same size now ... i do not know how to tell what they are but mine are mating (even after i called them bert and ernie :O) ... you can get a boy or a girl, whatever your preference ... i would probably buy one similar in size ... i have two in a 2.5 gallon and they are quite happy because they are active, even singing and mating :O)