GAL synchronization

My company is running Exchange Server 2003 in native mode. We acquired a company running Exchange 5.5. The intent is for the companies to operate independently in their own domains. However, we would like to be able to merge the Global Address List (GAL) data. For example, in Microsoft Outlook I should see an FRANCE   GAL (Exchange 5.5) and an CANADA GAL (Exchange 2003). This data should, of course, synchronize as changes are made. What's the best way to make this connection?