I'd never thought about this...but assuming that you just want to image the primary mmc and back it up on the secondary mmc (then you can copy it off onto a PC)

dd if=/dev/mmcblk0 of=/media/mmc1/backup.dd

and to restore

dd if=/media/mmc1/backup.dd of=/dev/mmcblk0

You would need to do a restore as root, and I imagine it would freak the tablet out quite a lot as well.
Anyone know if this would work ok?