asterisk users - Dec 2004 - switch statement.

Hi all,
I'm a bit confused about how to use the switch statement.
I've got an IAX2 link between 2 servers (SA & SB).
I have use the switch statement to include extensions from SA onto SB which is 
happening perfectly. I've read that I can't use the switch statement the
other way (ie SB->SA) at the same time.
What is the correct way for both servers to know about each others dialplans ?

TIA
Jon