If I read what your are saying correctly, you want to run two locomotives of on decoder.
Others may correct me here and I am happy to be if needed, but running two locos off one decoder would only be possible by using jumper leads from one unit to the other. While I think
it would be possible, why would you bother?
For the cost of a decoder it would be far easier and, to me logical, to have each locomotive receiving its own signal.
After all, consisting with the NCE equipment is very easy and takes only a few seconds.
I am not trying to put the idea down, out of hand, I just can not see the long term functional arrangment being there for you. Even as a test bed the time and money it takes to put in a TCS (or something of the same elk) decoder would be better than having to risk shorting something out and destroying a locomotive. I have had a decoder catch fire and melt the side out of a loco due to a short in the decoder. The loco had been in service for some time before it happened.
I would go down the path of using two decoders, as you also suggest, and still program them to each locos' number. As I have mentioned, it only takes a few seconds to set up a consist. Perhaps a little longer when you are getting to know the equipment, but certainly not long enough to warrant any other modification. imho.
NCE gear is very easy to use and I have a common rail set up and have had no trouble for quite a few years now. Something like twelve years.
As I said, I will stand corrected if need be.
Regards and respect