|Your post is a little confusing.|
Just so you know, if you put a second NIC in a computer and one NIC has internet access and the other doesn't, the computer itself still has internet access.
As to your question about a second secure network, click on my name above in this response and read my “how-to” guide titled, “Add a second Router to your LAN”
You'll want to pay attention to the scenario where you interconnect the two switches "LAN port to WAN port" and are using a different subnet on the downstream router.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.