You'd only really need two such cards if you were looking for a way to dump your own games into ROM form, and/or for some wacky reason can't find it available through other means. There was a homebrew way of being able to dump games and save data over wifi to a PC and back, but that I haven't used it for a long time, so I don't know if it's been updated or abandoned since.
As for the DStwo, [url=http://eng.supercard.sc/manual/dstwo/]their site at least has the latest firmware, EOS (basically the loader/OS environment), and some of their own homebrew for the card, so you can get an idea of what it's capable of. From there, it's almost literally just a matter of putting the contents where you want/need them to go on the microSD(HC) that you use.
To date, I don't know of any DS flashcard that is compatible with SDXC... maybe if someone manages to make a working 3DS flashcard that would be possible. Even so...
Good luck all the same, and enjoy. --
Because, f*ck Sony