By default, SQL Server Express is not configured to accept remote connections, which helps reduce the risk of attack via the SQL Server service. If you do need to access your Express server from other machines, you can enable remote connections using the Surface Area Configuration tool.
You will probably need to open the TCP port your Express servers are listening to in your firewall as well. The default port number is 1433.
client to client connection
Andy Kemshall
You will probably need to open the TCP port your Express servers are listening to in your firewall as well. The default port number is 1433.