asterisk users - Jul 2006 - how to enable users on other iax server call my iax users

Hi,

I have two iax server, one is asterisk on ip 192.168.18.8, other is 
freeswitch on ip 192.168.18.180. I wanna asterisk iax users could accept 
or call freeswitch iax users. How could i do it in asterisk configure?

Bests,
Bobber Cheng