asterisk users - Sep 2005 - IAX2 registration

Hi!

I have the following setup:
PSTN1 <----> Asterisk1 <--- IAX2 ---> Asterisk2 <----> PSTN2
As you can see, two Asterisk machines are connected via IAX2. There are users 
connected to each Asterisk machine over a local LAN. Each of these users in 
both LANs should be able to either use PSTN1 or PSTN2, depending on which 
extension they dial, no problem here.
My question now is, does each Asterisk server need to register with the other 
Asterisk machine, or is it sufficient if only one Asterisk registers with the 
other one? Will the other one then know about the connection and be able to 
make phone calls over that one?

Thanks,
Christoph