This is an interesting little trick that I've taken for granted for some time now and didn't realise a lot of people weren't aware of it.
If two adjacent walls are within say 25mm of each other and you use join geometry between the two walls (they don't even need to touch) the two walls inherit the openings of the other.
This is very handy when you are forced to model finish layers separetly in some situations. Saves you doing the unthinkable and using the evil "edit profile" option...
HTH.
