Allison DID take a Merlin two stage supercharger and stick it onto a V-1710 and found it worked fine - for a little while. Note that unlike the Allison supercharger, the Merlin supercharger did not come off the engine without resorting to a saw and it turns out it did not run well when not part of an integral crankcase, failing pretty quickly. As to why Allison did not at least add a second speed to its single stage supercharger, I have no idea. It would have required a new rear accessory case but I think that would have been easily possible without a problem shoehorning into the P-39, P-40, or P-51 airframe.
And note that for the P-63 and F-82 Allison did develop an auxiliary stage supercharger to replace the turbo and couple it to the original accessory case via the starter interface. That would not have fitted onto a P-39, P-40, or P-51 without significant airframe changes, since it added quite a bit of length to the whole engine package.
Fact is, a V-1710 with a GE turbo was a very capable package and in general turbocharged engines gave better performance than mechanically supercharged equivalents. And as a someone who took too much thermodynamics in college I'll never understand why no one but Stanley Hooker thought up using a liquid cooler aftercooler, which solves all kinds of problems compared to an air cooled equivalent. The fact that Sir Hooker was a theoretical aerodynamicist and "not much of an engineer" but came up with that brilliant solution while literally legions of mechanical engineers did not either develop or copy it merely emphasizes the man's brilliance. It was that aftercooler more than anything that restored the Spitfire to viability and enabled the P-51 to show up over every enemy capital. The P-38 suffered from various intercooler problems and a liquid intercooler could have fixed those with minimal, but never underestimate the power of stupid people in large groups.
And for the F-82 at Air Force orders NAA abandoned the proven Merlin installation of the P-82 and went with a boosted V-1710 WITHOUT an intercooler and as a result built an airplane that almost had to be abandoned. Note that SECAF Symington was a former GM executive.