Ahh I thought they could communicate with each other, that makes the problem somewhat more difficult
. I suppose what you could do is transport a SQL Backup of the remote database (which would be quicker and smaller than a standard SQL Server backup). Then locally or in the network of the main database perform a comparison restricted appropriately prompted via some service or scheduled task. Then the remote machine could pick up the synchronization sql generated and run it locally to perform the update.
Does that make sense in your environment?