Your post badly mixes up players - as in the real people at the table - and PCs, the characters in the fiction. This is quite relevant if-when a DM allows a player to run more than one PC simultaneously in a party.I'm considering running two separate groups through Storm King's Thunder at the same time and they would cross paths a few times, but pretty much end up playing the campaign in parallel. The combined sessions would end up with 7-8 players though. Ugh, that's a lot. I prefer 3 in a group. 4 max.
So, for anyone who's run a large party...what are the secrets? How does each character get a fair amount of screen time?