Instructions
Select a Pokemon that you do not need and are willing to sacrifice in the first game.
Give a Pokemon a Rare Candy to hold in the second game, and save your progress.
Connect the two games using a link cable.
Trade the sacrifice Pokemon for the one holding the Rare Candy.
Save the game on the first Game Boy, where you have just received the Pokemon holding the Rare Candy.
Turn off the second Game Boy, the one which has just received the sacrifice Pokemon, without saving the game.
Start up the second Game Boy again, and the game will still be at the point where you have the Pokemon holding the Rare Candy, prior to the trade.