Checking if user authored an entry in another channel while in loop, N+1 problem
There's one bit missing in the constrains you explain above, do users end up with 1 entry only per channel so a total of 2 posts max?

If so, we know that:


each users can have a max of 2 posts
a user cannot post in B is they haven't completed their post in A


So assuming this first condition is also true (2 entries/user) you could just loop through users post count looking for those with less than 2 posts and they would automatically be the users who haven't posted in B..? Just a shot in the dark :)