1. Make sure they are both of the same type (NiCd or NIMH). If they are stock then they will both be 50mAh NiCd's.
2. It'd be nice for them to have similar capacities (or else you'll just be able to charge up to the limit of the cell with less capacity and waste the potential of the other).
Other than that, go right ahead. You planning to make the donor bit rubber band powered or something?

IMO, if you're going to go to the trouble of doing a dual cell mod, get a couple of 150mAh NIMH's from the shop here and do it right.