[2020] BMP-3 & BMP-2M. Missile fire when main gun is firing.

There is nothing to fix here, it's just your controls setting that is wrong.

I just checked BMP-2M. I can shot gun with one button, launch missile with other and control it during flight without problem.

I can launch missile and during the missile flight I can fire cannon (obviously, when I move cannon, missile will also move due to fact that it's manually controlled).