I dunno. WE do a ton of backpacking, and the only thing we double up on is water, sunscreen, bug juice---the kinds of things we might need right there on the trail and now want to interrupt the hike to get them from the other person.

WE split our weight based on body weight. She weighs about 65% of what I do, so she carries about 65% of what I do. Works great. I carry the food and tent, she carries the "kitchen" :stove, pot & bowls.

One thing we each carry is an emergency whistle. We have only used them twice, but in each case we could hear the whistles when we could not hear each other yellling. Very cheap, and we won't go on a trail without them. I carry mine in the camera bag, and she carries hers tied to her shoulder strap.