throw the ball on the first pokemon.
on the seconds turn, switch the position of the master ball from first to second using the y button.
he throws the ball and on the next turn, it is still in the pocket.
after the battle in the open switch it cack to first.
Sell the second item now(make sure you only have two slots)nothing after it.
Whenyou look at the pocket it shows you only have one but it will not go down.
Then you make a pokemon hold it and trade it to the game you need and keep doing this