filtered? if not then 50% water change half way through the week and a 100% change at the end of the week.. if its filtered id go for a trio of frogs with 25/30% water changes weekly.
We have two but they don't ever seem to interact together or even hang out together. I don't know if they prefer company on the whole, but ours couldn't seem to care less if they see the other one from one day to the next.