How about a picture of your trailer?
I have never seen a T NEE with out two tailights. T Nee did have what they called a light bar that was a piece of angle iron mounted across the frame about eighteen inch behind the fenders. I have a picture of this I will try and find it. Otherwise if it was me I would put two tailight on just to stop some one from stopping you to tell you hey one of your tailights is out. Sure makes it safer to change lanes on the road too.
If you're going to use it on the road I would wire it up with two lights, one on each side. Make it with functional brake and tail lights, or you stand a good chance of having issues with local law enforcement folks.
I've heard that if the vehicle originally only had one tail light you could run it that way, but personally I wouldn't take a chance. Safety FIRST is my way of thinking on this. Just my humble opinion........
Edit: Guess Dell posted while I was typing, we're on the same page here...
It probably only came with one tail light, back then that was all that was required. As a matter of fact, the Ontario Highway Act to this day, only requires one tail light. But as pointed out, I would suggest wiring two tail/signal lights for safety.
I couldn't see Del's pic, but I think this is the light bar he's talking about