Setting up Two Exchange Servers

We are setting up a new windows 2000 server on our network. The other server is running Back Office Server 4.5 wth Exchange Server. The new server is a seperate domain from the old server. The servers each see each others domain, but not shared resources. Can someone provide me with a step by step or direct me to an example of how to set up the new exchange server so that users on both XS can email eachother? And does the fact that they see eachothers domain but not the shared drives on that domain have anything to do with it?