Looking into it I don't think they've implemented this for virtual MC users. I believe if you're on a Wii you can use a physical MC in slot b, otherwise it doesn't seem possible with virtual memory cards. I could be wrong though and one would think its just a matter of renaming the MC to MCSLOTB or something... but yeah I didn't find anything.
Just curious. Would you be able to have a virtual memory card in slot A and a real memory card in slot B? Or would in order me to to use slot B I would have to turn off virtual memory cards and have a physical card in each slot?