The main problem isn't file locking, I'll need to poke and prod whoever told you that.
The main problem is that we use the Members directory for much more than just member records. We keep post histories and indexes, private messages, and permissions there as well.
In short, you can't have private forums at all on either board. If you do, the forum numbers are going to end up in conflict, and changing member profiles on one will end up doing fun stuff, such as removing private forum rights on the other.
You won't be able to do any member based topic searches because the post indexes will conflict with each other.
Don't even get me started on private messages...
It's bad mojo, don't do it.