When I can mine both BTC and NMC at once, the linkedness falls apart completely. Suddenly, if I want BTC, it's best I mine both and exchane the NMC. If I want NMC, it's best I mine both and exchange the BTC.
So now everyone's mining both, so the difficulty becomes equal, and since the miners no longer look at the difficulty and exchange rate of both, and they are no longer linked by difficulty, only by how many people want BTC and how many want NMC.
I read this previously but didn't think it made sense. After a second reading, I think I'm seeing how it might decouple the value of NC to BC but it's still not crystal clear why people won't start equating NC to BC since the cost to produce becomes identical.