When my wife and I hike together, I carry the tent, water filter, and a lot of food, and she carries the kitchen kit and some of the food. Funny thing, our base weights are about the same -- my pack weighs a LOT less than hers empty. In any case, we don't worry so much about relative weights since our total pack weights are pretty light.

It's a good idea for each hiker to have a small first aid kit, and to carry snacks for the day. Water treatment isn't such a big deal -- in an emergency, just drink the water. It'll take several days for any bugs to get you, while dehydration can make you miserable very quickly.