1 stepper motor driver using two motors
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.