I don't know the precise answer. The multi-wan doc is here and it looks like the solution may be a source based route in the Multi-WAN module. If you really wanted you could also use a second LAN NIC and put the web server in a DMZ.

Two routers to one

I have two static IP addresses/internet connections. One is for the business side that all the workstations connect to, and the other is only for the web server. I currently have one line going to my clearOS 6.2 box, then distributed through the network. The other line is going to my smoothwall, then straight to my webserver. The webserver is "multi homed" and has a second network card that goes to the main lan so we can render images to the webserver for customer downloads.
I would love to take this down to one clearOS 7.2 box and have the same functionality.
What would be my best route for accomplishing this? Would multiwan need to be involved since they arent being load balanced? I want the webserver line to just go straight through with firewalling how it currently is. Some IPS in there would also be nice.