Wanting to connect GW6 and GW5 systems without merging

[Ben]

I have a client with two GroupWise systems (GW6 and GW5) that are
completely stand-alone today. They would like to "link" the two
systems together for resource scheduling, free-busy searching, and "if
possible" doing Proxy across the two systems.

If my GW knowledge is current External GW systems synchronize only
Address Book information and that the above functions can only be done
by having the two systems merged into a single system. Further, the
Primary Domain needs to be GW6 if any Secondary Domain were going to
be GW6.

Can anyone verify this or let me know where I am mistaken?

Thanks.