|Using a second computer would solve it, but that's not ideal.|
Life's isn't always ideal and sometimes you just have to go with what you can do, as compared to what you want to do.
The only way I can think of doing this is if your 'external network device' is a router. You could then create two subnets, route between them and put one NIC in one, the other NIC in the other. Then pass data through.
If you don't mind me asking, just what is it you're testing?
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.