Let's say I join a group, provide space on A, and get links to B. Do coop members know that A and B are controlled by the same people? How about outsiders?
Domain registry lookup...coop members for the most part wouldn't care -- just don't interlink A and B to avoid any potential SE problems.
As the saying goes all things with moderation. It is my belief co-op member s use their weight to run ads on site A and point ads to site B all the time. I would not want co-op links to be only links pointing to site B but those links will be running on other peoples pages not just the pages of site A. If not having site A and B connected to you in any way, consider using private registrations. Shannon