Exchange 2007-2013 Co-existence breaks Free Busy time between servers

My company is in the process of migration from Exchange 2007 to Exchange 2013. We have used the recommended path of giving our 2007 server a legacy domain name and pointing all initial CAS traffic to the new exchange 2013 boxes. We have a few mailboxes currently living on the exchange 2013 server, but most are still on 2007. Everything seems to be working correctly except being able to view free/busy information cross server. Users can only see free/busy calendar information of other uses that are on the same mailbox server, i.e uses on exchange 2007 cannot see calendar information of users that were already migrated to exchange 2013 and vice verse. This is going to present a problem when we begin migrating more users it will prevent them from being able to schedule meetings as they normally would.  I'd be happy to provide any needed configuration information.