Use the router's IP address as visible on the WAN/LAN (ppumkin: outside of the setup)
You should create two routings on the router so that connection to one port is mapped to the VPN port on one computer, and another port is mapped to the VPN port on another computer. Then you can VPN to your two computers by specifying the router's IP address and a port number of your choice
Update: Here is a guide on how to forward ports for the BitTorrent app. You will have to similarily go to the "Virtual Server" tab and set up port forwarding:
Set up two distinct public ports, so you can connect to two different computers from the outside. As for the private ports, make them match the port used by your VPN application. The private ports can be the same.