Shadow is right, there is not really a concept of 'Main' computer. It should be both ways (or all ways for more then 2).

Also, I can see, though not tried or tested, a bug in that if you start a MLB Instance on computer 1 and enable broadcast or Key filtering, and then start the next MLB instance on computer 2, then they broadcast and Filtering will not be synced. The toggling will be wacky. Enabling one will disable the other, and visa versa.
Again, I have not tried this, but knowing how the code works, I am sure this happen. I will probably need to look into putting in some sort of sync that gets all Clients on the same state.

Just something I was thinking about while setting on hold with Blizzard for the last 40 minutes.