When both species of frogs are small tadpoles, I'll feed them boiled pieces of lettuce and when the Bullfrogs start getting bigger than the treefrogs, I'll seperate them in 2 ten gallon tanks.
I will then put about 6-8 treefrogs in 1 ten gallon and feed them small crickets, and I will use the other ten gallon to put in one bullfrog adult and feed him/her feeder goldfish that have been put out of water. Seeing a beached, flopping, defenseless fish is like saying "Come eat me!" to a bullfrog.
(The rest of the frogs I will release into the pond where I got the treefrogs at, since I already see them there.)