You will need at least two ds to do anything with the two games at one time. If you want to trade pokemon you can try something but its ** INCREDIBLY** risky as you can lose the pokemon you are trading over. First you need wifi. Go to GTS and deposit the pokemon you want in the other game.Put up a pokemon you know you dont mind trading away to the other game.After depositing your pokemon quickly switch the games and enter the GTS from the other game. Search for the trade you just put up and trade with yourself. This is risky as anyone can get on GTS and take your pokemon thats why you must be quick.
Example: i put up a zekrom on GTS from my white.I then switch to my black and trade a pokemon i selected for that zekrom. Easy enough