if you put both secondary and special weapon select on the same key the secondary select will get overruled and you cannot use it to switch to secondary weapon whether your vehicle have special weapon or not
solution i found
is to bind it with the same key as primary, it worked on observed vehicle
(`) = tilde key
vehicle observed:
BMP-2M : both grenade launcher and and auto canon can be switch with both key (Tilde key) Grenade launcher (Left Ctrl) Auto Canon
BMP-3 : both missile launcher and auto canon can be switch with both key (Tilde key) Missile launcher (Left Ctrl) Auto canon
BTR-82AT : both missile launcher and auto canon can be switch but it is inverted auto canon now is controlled by secondary weapon selector (Tilde key) while missile is controlled by special weapon selector (Left Ctrl)