I am using 2950, on which I was trying to run (built-in) SSL VPN service through WAN1 at port 443, but at the same time want to run another HTTPs server BEHIND the firewall through WAN 2 at port 443. Is that at all possible? If so how should I config the router? Thanks.

I don't think that this is possible.

I had a very similar issue with the SSL VPN on the 2950 - we host a bunch of SSL services behind the 2950, and when SSL VPN is being used (or even if the default port is not changed from 443) it intercepts every 443 request (completely overriding Port Forward rules).

Support came back saying that the SSL and HTTPS ports had to be the same on the router (both of which step in before NAT rules) - this alone (and the fact that it hasn't been enhancede) meant we had to drop using Draytek for SSL and instead use an MS SSTP Server), bit of a pity really as it is not mentioned in the documentation.