connect to server with IP address running sql-express instance through remote desktop

I have my VPS server in a remote location with ip address 172.16.11.2 (for example) having two instances of sql-server express running.

one instance is Microsoft SQL Server 2008 R2 (SP2) where I connect from my local PC using remote desktop with the following credentials:

 servername : 172.16.11.2
 authentication : sql - server authentication
 userid: xx
 password:xx

the other instance is:

Microsoft SQL Server 2008 R2 (SP2) with advanced services. To connect to this machine I remote desktop to the server and type in

 servername : machinename\sql-express
 authentication : sql - server authentication
 userid: xx
 password:xx

now using remote desktop from my local machine how will I connect to this instance of sql-server.

I tried

 172.16.11.2\sqlexpress 

but this failed.