I have 2 NIC, eth0 has IP=192.168.0.34 and eth1 has IP=192.168.1.39
Here are my notes on add a 2nd NIC (eth1) with a new IP and different gateway from eth0
Default gateway will still be 192.168.0.33, meaning traffic originated from the machine will go out this interface, but traffic replies will go out the same interface that the request came in.