Exchange Setup

Hi

At present I have two independent forest, each running Exchange 2003. I am moving towards having a single domain over two sites. Each site would be running Exchange so that the WAN link between the two offices would not be saturated. One office has 35 employees while the other has 15. I would like the main office, 35 people, to be running the primary Exchange server and the branch office, an Exchange server as well (backup purposes etc). What would I need to do to consolidate the two servers so that they are running under one Exchange organisation.

Thanks