Sorry, I can't speak for the N900 but I used to have a multi card reader that showed up as two drives and only one appeared under linux. The problem was a kernel option called something like "SCSI: Probe multiple LUNs" needed to be enabled.
Failing that, why not try "mount -o bind /sdcardmountpoint /home/MyDocs/sdcard" on the n900 so you can just access the sd card from within the home directory.

EDIT: I've just noticed you run gentoo. That's what I used to run, and probe multiple luns isn't in the default kernel config, or at least it wasn't two years ago...