If I read your post right, it should have not melted. I am interpretting the the hot and neutral were landed on their respected hot/neutral screws then the wires from the second drop cord were landed on the second set of screws of the melted receptacle. If that is the case, then the outlet should have not melted like that. It is difficult so tell from the 2 dimensional picture. I would suspect a loose connection or a possible charging issue with one of the trucks. I agree with sp4rx that each truck should be on dedicated circuits. If the circuit failed and was not caught, two trucks could have been KO'd instead of one.
If a non-sparky was responsible for such an important conncection, I would start back tracking his or hers handy work. Remember that firefighters has a pyromaniac for an inner child.
