How to measure stepper motor current

Having just addressed this myself I'd have to say separate drivers for each motor is the most sensible approach.

If you remove the power from one motor, is has no holding torque and will 'flop' under any force.

You may also have issues with the state of the outputs not being correct for the current rotor position, which will make the motor jump a bit when you switch it.

And as Dave mentioned, if you have power on a coil when you switch it, it will arc inside the switch which will eventually destroy it.

It's also way more impressive to have both axes moving simultaneously if you can :)

Your application may be able to cope with some of these issues, but if you need reliable accuracy then I'd recommend separate drivers. The A4988 (update of the A4983) is only $3-4 from Digikey if you're building your own board.

At 12 V the speed will soon be zero since something will overheat and break. That's if this A4988 thing can even supply the necessary 7.5 A per phase. If not, then it will probably get hot and break. Either way, this is not a good idea.

There is one exception to this, which is if the 12 V is only applied for short periods of time to overcome the inductance of the windings, with the voltage then quickly brought back down to spec before the current exceeds spec. That sort of drive can be useful for steppers because the current in the coils switches faster, which allows the motor to run faster. However, care must be taken to not exceed the rated current. Unless this A4988 thing is specifically designed to do this and you can set a current limit at the 1.78 A maximum the motor is rated at, the points in the first paragraph apply.