Is your current desire to enable Western Hemisphere users on the Eastern Hemisphere LCS deployment? If so, you can use ILM to create contact objects in the forest with LCS and then populate the MSRTCSIP-OriginatorSid with the objectSid attribute of the Western Hemisphere account. This is documented in Deploying_in_a_Multiple_Forest_Environment.doc, which can be found in the LCS 2005 resource kit.
If you want to have a separate LCS or OCS deployment for the Western Hemisphere then you can federate the two systems until the point that you are able to merge them. The down side of this method is that you won't have a unified address book.