There is not enough information stating what it is exactly that you are trying to do. You have two forests, each with its own Exchange Server and you would like for them to “communicate”. To facilitate communication at the most basic level, that is sending an e-mail from one Exchange Server to the other, each Exchange Server must be able to resolve the DNS and SMTP name space of the other. You can accomplish this using DNS secondary zones in Windows 2000 and prior or DNS stub zones in Windows Server 2003 or greater.
Taking it a step further, such that users on one Exchange Server are able to select users from the other in the GAL and vice versa, requires synchronization with can be accomplished using GALSync, part of IIFP which is a free tool from Microsoft, ILM/MIIS or third-party directory synchronization applications.