Although this idea is most immediately relevant to people who have children in multiple sizes of diapers, the general concept is helpful for anyone who ever packs anything:
When D2 was born I started keeping one Ziploc of diapers and any other necessary changing apparatus for him, and a different Ziploc for D1. Thus I could see at a glance how many diapers of each were there. Two more bags held the spare outfits for each. Yet another bag for a snack, if needed, or for blankies and nursing drape, and not only could I actually find things in the diaper bag, clean things stayed clean. Also, if they were going to different destinations I could easily split up their supplies.