I tend to agree with the previous post. You would have to switch off the stepper driver when changing, or add a circuit to better absorb the back-emf from disconnecting the motor. It seems impractical to me since how do you switch between motors accurately? When one motor is switched off and the second one switched on, the second one is likely to have to start on a micro-step position, so the driver would have to somehow start the motor without the rotor moving, even when the rotor isn't aligned with a pole tooth.

Are you sure you get the machine fast enough with the right motor and 1:2 for high resolution and do you really need a 1:2 ratio? A servo motor would make this easy.