it is possible to create a shared server group on server 1 which contains all users of server 2 (acutally you need to all all users of server 2 manually and all remote users or entities should accept presence subscriptions automatically.) and display the group for all users (also the users on server 2).
So it is not exactly what you want and I wonder why one does not use the search function to find users of server 2.
but i decide this problem by using next solution : for 1 user on server 1 i share all groups on this server, after that i am export the contact-list from user1 to *.xml file and then import this list to contacts for user 2 ( on server 2) ! It was an easy solution, but need some time to think about it.