I know I don't have 2 but if I did then I would do hand me down undies. They aren't really 'sharing' cause they are clean, like you said, and they fit both of them. I am sure at some stage they will get mixed up anyway.

I get a lot of hand me downs from cousins and friends (thank goodness) but I don't take undies... probably doesn't really make a difference but I think it's ok between siblings.