Step 1 - Ensure that both network connections are enabled and functional
Step 2 - Right click on the network connection that is used for the internet, go to Advanced Tab, check "Allow other network users to connect through this computer's Internet connection", Click "OK"
That's as straightforward and as simple as it gets.... The second network connection is then assigned a static IP address and any pc that connects to it, with DHCP enabled, will get internet access.
Of course a cross-over cable would be needed to connect the two PC's. But a switch is even better.
With a switch, just plug in the Modem/Router to one port on the switch, then plug in the other PC's... once the internet device is DHCP enabled, then the other pc's will get an IP from the router and voila, internet! No need for two NIC's...
PS - Beware Windows Firewall or any other Firewall you may have installed, they can severely hamper this "simple" matter