After all these years I should know the answer to this but...

If there is a fairly large but still legal size group, must they occupy
a single campsite or can they split up on two sites?

This was asked, but I don’t want to just give my opinion. Is there a regulation?

I would think each group would need a separate permit if they were camping at different sites.

walllee: "I would think each group would need a separate permit if they were camping at different sites. "

Sounds right. Otherwise anybody could say" I'm with the group that's camped at the next lake and they have the permit.

Your group could split up at anytime during your trip for day trips or fishing different areas. This is why you get a main big permit for the trip leader, and smaller tear off permits for each of the other canoes. Each canoe must have the permit with them when traveling. The multiple smaller permits tabs of the same permit do not allow your group to occupy multiple campsites. Your group each night must return to your main campsite. Many trips I’ve been miles away from our group on other lakes fishing, but I returned to our main camp at night.

One permit, one campsite.

One campsite is permitted per overnight paddle permit.

https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3799760.pdf

If part of your trip plan includes not being together, you can get two permits. There is no rule against having two permits at one site as long as you still obey the 9 people and 4 watercraft rule. We’ve done that both for going in together and splitting up and also entering on different days and following different routes before joining together.

Not for camping. From cycle003's FS link, BWCA Wilderness Rules and Regulations Section, Campsites: • All members of a permit group must camp together.

cyclones30: "One permit, one campsite. "

It's not that hard, if you have one permit you are allowed one campsite.

As has been stated, one permit one campsite. The quotas are partially(maybe mostly?) based on campsite availability in an area. There is no way to control this without prohibiting this practice.