Hello and welcome to FF! Sorry you lost one of the frogs . Sadly, most frogs will eat other frogs that fit in their large mouths, including their own kind. What species of Green Tree Frogs are you keeping, the American Hyla or the Australian Litoria? Depending on the frog species, you will have to provide it's temperature and humidity needs. You can house multiple, same species frogs, in the same enclosure as long as they are close in size and the enclosure is big enough to house all.
Recommend not moving tree frogs to separate container to feed. Better is to use a small glass bowl and place the insects inside; they won't get out. Once frogs learn to eat from it they will associate the bowl with food. Remember to use food supplements: http://www.frogforum.net/food-feeder...schedules.html.
A 20L is not a good enclosure for tree frogs unless stood on it's side after a vertical conversion. Tree frogs like to climb and 24-36 in. tall enclosures are recommended. Good luck !