I haven't tried this specifically (and my tablet's in for repair), but I have one suggestion: leave both MMCs mounted, then make a folder in one of them, and mount the flash drive there (e.g. mount /dev/whatever /media/mmc1/foomount). That would be simpler, in any case, and wouldn't leave you with the same problem for the MMC. Also, if you give us the error you encountered, it would help...