In the link I pasted they guy actually forwards it to his primary extarnal IP, so probably not what you are looking for.
You could also create a loopback interface, and assign any internal IP to it, like 10.40.1.1/30, or just a /32 as you only need one IP
Then create a VIP address with your second external IP and forward it to the IP you specified for the loopback on port 443
Then in the VPN settings you select the new loopback interface as the listening interface. I have done setups like that for IPSEC VPN so I am sure it should work for a SSL VPN setup.