But surely the original question concerned one stepper and two drivers? And that's the question that's been answered - best way is one driver, one motor, and switch the inputs to the driver. However, the precise way of doing this - couple of logic chips, relays, switches, etc - depends so much on what you have already. Need to isolate common grounds, whether opto-isolation is needed, a bunch of things like that, are trivial to sort out - but only with a full understanding of where you are coming from, and I appreciate that this is difficult for someone with no knowledge of electronics so no insight into what info is needed.

But please don't change the question!