Hi, If you use the built-in availability service, you must configure GALSync (to represent mailbox users in the remote forest as mail-enabled user [MEU] objects in the local forest) and use the Add-AvailabilityAddressSpace cmdlet to add the respective namespace. You should also note that each Exchange forest should be able to connect to the availability service in the other org using the Fully Qualified Domain Name specified for the Internal URL of the Exchange Web Services virtual directory. You should establish a forest-wide trust relationship between the forests. http://www.msexchange.org/articles_tutorials/exchange-server-2010/migration-deployment/deep-dive-into-rich-coexistence-between-exchange-forests-part1.html

Here is link, describing, exactly what you are looking : http://technet.microsoft.com/en-us/library/hh310374(v=exchg.141).aspx