I think there's an important material difference between the two. China's single party is authoritarian and uncontested. America's two major parties are mildly authoritarian on different axes, but average out to a mostly liberal status quo in practice. The relative chaos and transparency of America's system are what they are, but it isn't an autocracy at this point.
There's also a significant growing political push to transition away from FPTP voting in the US, which would dismantle the current duopoly.
There's also a significant growing political push to transition away from FPTP voting in the US, which would dismantle the current duopoly.