But, I disagree with your assessment. Why would only a few people have had both? I knew far more people that had both than had one or the other. This is 2 separate game generations, the NES came to the US in 1985 and Europe in 1986, while the SNES came to the US in 1991 and Europe in 1992. That is a full 6 years between launches. Even buying on the end of the life cycle, it is reasonable to think that same person would/could upgrade later to the next generation.
Now, Sega Master System vs NES or Genesis vs SNES, that is more likely to get one or the other. Most people do only own one console family per generation.
I had both. I actually still have my NES in the cellar with about 20 games and the light gun to go with it. Unfortunately, none of the games are valuable.
A pawn shop in my area picked up a box of old NES games somewhere and in the box was Flintstones at dinosaur peak. This is about an $800 game. Some people have all the luck I guess.
What's funny is the pawn shop guy tried to sell the flintstones game to an independent game store where I live and wanted $800. The owner told him to get lost. Guess the pawnshop guy never heard of ebay and paypal fees.