Different cameras allow you to do different things with memory cards and folders so it kind of depends on the SLR you chose.
It would almost certainly be easier to each keep a memory card in your pocket and swap them when you pass the camera over. I take you mean for when you both use the camera at the same time otherwise you just need to clear the card after each use.
The other question is, do you really need to keep the shots apart? Does it matter who shot what?
It will be very difficult to take the credit for her better shot is she can prove she's the artist. ::