You must use Pokemon Bank (Pokemon Transporter).
You only need one 3DS but it must have Pokemon Bank paid for already, and PokeTransporter (which comes free with it). Just load PokeTransporter, put B/W in the slot and transport the Pokemon you want into it. Just a warning, you can't transfer it back. Then, load Pokemon Bank, put your desired game in the slot and take the Pokemon out of storage into your boxes.