I have one for each - it really helps when DD goes to my mum's while I take DS out to the Dr's (or vice versa)...It means I don't have to make up another bag for one when the ocassion arises. On the pusher I have two hooks - one on each side where both bags hang for stability.

DS has a blue Quiksilver backpack and DD has a pink Billabong backpack...that way not even DF can mess it up .