From Mach3's point of view the system will not look any different regardless of how you power it. Mach3 just sends out the step and direction signals.

My post here may interest you, since it includes more detail about why 24V (TB6560 etc) can be unsuitable:
http://www.mycncuk.com/forums/genera...html#post25191

So long as the PSUs are electrically isolated you could connect them in series as you describe, so long as the total current drawn from either is not greater than it's rating.

Before I comment further, can you provide a link to the datasheets for the stepper motors you have since that would make it a lot easier to work things out?