No, the slave axis is always slaved to the master, whether you have switches or not.
I believe what you are referring to is auto squaring the gantry. If you use a separate home switch for the Y and A axis, then Mach3 will unslave the axis while homing, allowing each side to home to it's own switch, squaring the gantry during homing. The axis are then re-slaved after homing.
But if you only use one switch, or no switches, the slave is always synced with the master.