Heheh, well let's say you have several pokemon games, like Diamond and Soul Silver. And you want some pokemon from Diamond to be in Soul Silver. So you just ask someone to trade with you xD first you give the pokemon from Diamond to the person, log off, insert your SS version and the person trades them back to you. 0_0
in our case you must have two junk pokemon with you that you don't really need. by the way, which games do you have?