Bottom line is that the USAAF woefully neglected fighter development, not least by refusing to require 2-speed supercharger being but one glaring example.
There are two speed superchargers and there are two speed superchargers (and the engines they are attached to).
For the US there were only two viable fighter engines
ready to go and in production in 1939. The Wright R-1820 9 cylinder and the P & W R-1830 14 cylinder radial.
Allison delivered 46 engines in 1939 and 29 of them were for the twin engine YFM-1. Allison also had to take back and rework 277 P-40 engines in 1940 in order for them to make rated power and still meet the original overhaul life. Mucking about with a two speed supercharger was only going to delay something else.
for most companies using a two speed supercharger was a way to increase take-off power without hurting altitude performance.
Most 1939 engines had full throttle heights around 13,000ft (4,000 meters) give or take a bit (1500ft?) . The Merlin III was far and away the exception.
The superchargers were maxed out at around that altitude and spinning them faster with a higher gear wasn't going to get you much. The impeller tips were already getting close to exceeding the speed of sound inside the supercharger.
Even with RR the big advantage of the Merlin X over the Merlin III was the extra 200 or so hp for take-off using 87 octane fuel rather than the extra 1500ft or so of full throttle height.
The US had gone to 100 octane before the British (and had gone through the intermediate 91/92 octane) and that allowed the Allison ( and the two radials) a bit of breathing room on the take-off ratings without using a 2 speed supercharger.
Please note that the P & W R-1830 was good for about 950-1000hp at 14,500ft in high gear if it had two speeds and the same if it had one speed and used the high ratio.
I would also note that in the P-36 the low altitude engine was the -17 and the experimental single speed high altitude engine (14,500ft) was the -23 version. The US allocated the -19 model number to an experimental two
stage engine that gave 1050hp at 17,500ft. The two speed production engine with the lowest type number is the -33 used in the P-66 Vanguard and some early non turbo B-24s.
I would note that the US had built more turbo charged aircraft than the rest of the world put together by 1939/40 (something like about 100) although that includes the 50 P-30 two seat fighters. The US was hardly ignoring high altitude engines or fighters. ( see the YP-37s, The YFM-1 was supposed to use turbos and the P-38)