Hey Steve. I will give it a go.
Your NW set is a (2) car set that has the center truck assembly to connect the two cars together with regular couplers on either end. The new car you purchased is not a (2) car set but just a single car. I believe that having couplers as you have on both ends is correct.
I have a few of these and the (2) car sets all have the truck in the middle with no couplers. My single cars are just that, single with couplers on both ends. Just add the single car to the consist as you would a regular freight car.