The bullfrog may try and eat the treefrog, I wouldn't suggest mixing species at all. Not to mention there is a slight chance they could be toxic to each other...
It's just an unnecessary risk.
Thanks for the quick reply! I didn't mean Green Treefrog, sorry to not be clear enough! I mean Northern Green Frog as in Rana Clamitans, a similar sized frog and sometimes found in the same ponds.