Link 2 exch 5.5 server

I have 1 exch 5.5 server at site A and another exch 5.5 server at site b. both server are connected to vpn and can ping each other by name and ip address. So network connection is ok. But the exch in site A and b has diff org and sites (2 totally diff exch - bu same ver and service pack). If I were to link these 2 exch so that I could share public folder, calendar (can see each other availability)and GAL with each other, is there a way to do it?

appreciate the feedbacks.