I have the same issue.
It turns out the microphone set I bought, some crappy eBay ones, are seen by the Wii as one microphone despite being 2 physical microphones connected via a single USB lead.
You could buy another audio hub to USB adapter and plug your other microphone into that. Where you'd get something like that though is beyond me.
One of my mics broke after a day so I just have the one now anyway.