I've got prefolds and multi-size covers (thirsties duos) for DS and DD. Fits them both well, so I don't have to worry whose stuff is whose. Prefolds are pretty cheap, and easy to use, so you could keep a few just for the diaper bag if you wanted....?
I've also just purchased two diaper bags for my big baby and my little baby. The bigger baby might be going to my mother-in-law's while the tiny baby came with me to run errands, or whatever, so I'd need two then anyways.