Moving files between different depots is no different than moving files between folders within the same depot, with the exception that the target depot must already exist. Using your example, and assuming "Depot-B" doesn't exist yet, to move "folder2" from "Depot-A" to "Depot-B", you would simply do this:
p4 depot Depot-B
p4 edit //Depot-A/folder2/...
p4 move //Depot-A/folder2/... //Depot-B/folder2/...
p4 submit