How to see different Gal’s

Hi

I Have 2 diferent exchange sites AAA and BBB
Both windows 2000 Server with Exchange Server 5.5 Sp4

I Want to the people in AAA site see and send mails to the people in the Gal of BBB site and the oposite too

The server’s are in the same domain and in the same lan.

Any option?

Thanks in advance