Accessing OWA and RDWEB on 2 servers from 1 ext. IP using 443 ?

Here is my situation.

We have 1 external IP address and used to have SBS 2003 and remote web workplace was fine on that server.

We are upgrading to Server 2012 and Exchange 2013 and RDWEB has to be on a seperate server from OWA.

Here is the config:

Server 1: Windows Server 2012 (DC), Exchange 2013, (https://server1.domain.local/OWA)

Server 2: windows server 2012 (https://server2.domain.local/RDWEB)

Locally both site work. The problem is when accessing from the external IP. I can only get to one of them (the one that has the port 443 assigned in the pix501 router).

So when I type https://external IP/OWA I get to Server 1 but I would need to also type https://external IP/RDWEB and get to Server2.

Since I can only have 1 port 443 in the router pointed to either 1 of the servers, how can I do this?