I am going to be building the D model very soon and I was wondering the same thing. Rockcliff actually helped me out on this so I can't take credit for it. I have already ordered and received my 4 axis driver and motor kit from Rockcliff and have tested the electrical parts. There is actually an option in Mach 3 called slaving which basically send the same step commands to 2 axis outputs at the same time so you end up with 2 motors that move perfectly synchronized to each other. For my dual X axis motors I am planning on using the X axis and A axis from the driver board to drive the 2 motors. If you search for slaving in the Mach3 manual you will see how to set it up. It is really simple. Hopefully you get this before you connect 2 motors to the same output from the driver board.