Worst case you might call it a minor action for each character, but I would say if they are adjacent then just call it a minor action for the taker, assuming the item is in the giver's hand. Even if it isn't, would it be more work for someone to take a potion out of the pack of a guy next to him than to reach into his own pack and get it out? Probably should still be a minor action.
If you want to toss someone something, then I'd call it a minor action for both and require an easy DC dex check to catch it. The hitch being how do you work that in terms of turns? You would have to essentially have the throw happen on one player's turn and the catch happen on the other's. This kind of issue also comes up with any other sort of situation (usually stunts) where two characters attempt to do something requiring action from each of them.