Simon that’s a poor parallel
Simon, if the left wing on an airplane works, why do we have a right wing?
No, it's not, it's intended to show how stupid it is to claim that masks and / or isolation are unnecessary. Both just provide a percentage of protection, and the percentages combine to improve the result.
The argument seems to be that neither are really required by virtue of both being requested, which is a non-argument beyond belief.
Brakes and seatbelts both contribute to improving overall safety by reducing the overall probability of any accident being fatal. The fact that both contribute doesn't prove that neither one is necessary.