sort of 4th axis for my Taig

I have a Taig mill with Xylotex 4 axes drive. One of the steppers is not being used, thought I'd add a 4th axis at some time.
Using One-CNC for CAM, SolidWorks for CAD, Mach3 for control.
As an alternative, I've been thinking of setting up a second Z axis, with the spindle axis set horizontally rather than vertically. This will give the option of clamping long skinny work to the table and working on the ends. For example box joints could be cut this way.
Hardware wise this should be pretty simple.

Is there any way to program this rather than renaming all the axes? Renaming axes would leave me with a different set of 3 axes, is there a way to use all four?

Thanks for any suggestions.

Elliot