I have a question. I am retrofitting my old  CNC router. My router has 2 heads on the xy axis and I have 4 drivers controlling xyza. My old router controller was DOS and I could set the offset for the A head inside the program one time and never touch touch it again. I believe it was WinCNC.  Both heads would operate independent of each other. I could router with one head and drill with the other Just by entering Z and A. I did not have to enter any other g code. 
How would I set up Mach3 to do this.

I hope I explained this well enough to understand what I am saying.

Sounds like a perfect application for fixture offsets.  Use G54 for the router head, G55 for the drill head.  Set the offset between them in your start-up script, and you're all set.

Regards,
Ray L.