You would set that up with your ROUTE. This is mildly advanced work that is specific to your network so I can only note it and never tell you exactly what the route commands will be.
That and once in a while you find the network to be invalid. That is you can't assign the same LAN range to both cards. Some have tried that.
Frankly I'd leave it as one LAN and one network card.
I've turned an old machine into a raid-5 NAS using Debian/OpenMediaVault. I recently became aware of what "network saturation" means while attempting to surf the internet and transfer large files back and forth to the NAS, over a single Cat5e connection. I recently bought a secondary, 10G network interface, and forced the NAS Shares to use Only the 10gbit link.
Everything works, but...My problem: Since I've set up 2x network interfaces (one 1gbit interface for internet only, one 10gbit purely for NAS stuff), I've noticed it takes several seconds to actually access the internet/web pages over the 1gbit link. It's like there's some kind of lag. It's like it doesn't initially know which network interface to use when I'm requesting a web page. Is there any way for me to force windows/my internet browser (chrome) to use ONLY my 1gbit adapter for internet traffic? Am I wrong in my assumptions on what the problem is? Anyone else have this issue?
Thanks in advance!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic