you need to use sip phones that register with both of
your sip servers.
An alternative is using usrloc db-mode=3 (db-only) and doing data
replication in the database backend (e.g. MySQL cluster or master-master
replication) for location sharing in combination with the Path extension
for NAT traversal:
http://osdir.com/ml/voip.openser.user/2006-03/msg00014.html
It's a bit more complicated to set up but works perfectly fine.
Regards,
Andreas