My guess is it just restricts buying one, though it really is just a hassle if you need to leave all fcs on alts that may be in another fc to buy a house on your main / just adds the restriction of give the fc to someone else for a second to "hold" it.
If it does mean the fc can't own a house since a member is part of two fcs with housing, that's really inconviniencing, and would probably just destroy more than it does good.