Well I use a 35L for a 2-3 day trip, as it works for me and the gear I have. I haven't done Whitney yet, but since I don't ever go solo, usually it works out that my partner carries the tent and I the bear can. I'm kind of a freak, so we never let us stray far enough that we would get separated (like I'm right behind him or vise versa, ALWAYS; one stops, we both stop), which is why in our case, we are confident in carrying these two, pretty important items, separately.
I would say it depends on how well you know your friends... If you feel they are prone to drifting away from you, I would keep your 55L pack and carry all your own stuff. If you trust them to stay close, I would break up some items so you all have lower pack weights. But again, I would make sure you trust them to stay within eyesight, because the minute you decide to split your stuff up, is the minute they or yourself could get lost, and someone is without food or shelter.
We also don't use hydration bladders in our packs, but we do carry one on occasion, just not inside the pocket.
Edited by yosey (05/11/19 10:09 PM)