(With apologies to u/chronological)
I believe the recent idea "Dual backpack" is missing a vitally important variant.
I propose a single frame type backpack attached to, or integral with, a linked support mechanism designed to span across the shoulders of 2 hikers and so distribute the load between
them. Such a mechanism would have dynamic joints to allow some freedom of movement between the two backpackees.
2 main variants are proposed. One for a pair of hikers who wish to walk next to each other and one for leader/follower use.
A 3rd variant covers weird people who wish to share the load but walk in other arrangements.
In addition the mechanism should allow the backpack to be positioned so the load can be proportioned unevenly, giving one hiker a higher percentage of the load.
For completeness I also propose a variant where more than 2 hikers can be linked so that groups of many thousands (a veritable plethora) can help carry the backpack.
If not covered by the original posting from u/chronological I also propose that a duality or plethora of users might also carry more than one backpack using the same single mechanism.
I claim this is a vast improvement (non-obvious inventive step) over more primitive mechanisms where a simple static pole is used between two carriers to suspend a load that is not a "backpack" but a crude bundle or helpless victim/lunch.
Beyond this primitive static pole variant I haven't done any form of prior art search so... that's life :-)