You could format both cards as ext2 and mount one inside the other (e.g., mounting the external card as /mount/mmc1 and internal card as /mount/mmc1/more). The only caveats are that, AFAIK, the "Internal Memory Card" and "External Memory Card" links in the file browser point to the first partition on each card, if this exists. In other words, you will have to set up the links to the cards yourself. No big deal. Moreover, Windows will not be able to read from/write to the cards.