Hi Chris,
I enjoyed your family Christmas video. I particularly like the wall your grandfather built. I have a western town my great-grandfather built in the 20s. These family heirlooms are special.
If your NW2 is postwar with an e unit, it should have a lever underneath to disengage the e unit. You wouldn't need to do any rewiring. Just get the train you want as the b unit running in reverse and while it is running hold it and reach under and move the lever. This will disconnect the e unit and keep it in reverse. Do the same thing with the other unit in forward and you will have your lash up.