The issue is that you can only have 1 default gateway.
Try adding a static route pointing to NIC2's subnet.
For example:
Assuming NIC2 is on the 10.2.2.0/24 subnet; the router for that subnet is at 10.2.2.1; the device you want to get to is at 10.3.3.3
At the command prompt-
route -p ADD 10.3.3.3 MASK 255.255.255.255 10.2.2.1
Note that you'll need a route back. Whatever the router is for the 10.3.3.0 subnet will need to point back along the same route.