I have a large DIY CNC router (5' x 10')  and want to slave X and A axis.  I have two questions?   (1) Do I have to have two homing switches for the X and A  or can I use one.  I don't want to rack my gantry.  Spent three days getting the linear bearings parallel within .001 over 10 feet. .   (2) The X axis is a rack and pinion so one motor will turn clockwise and the other counter clock.  How do I program this in Mach 3.

You can use just one home switch. There's an option for this in General COnfig, "Home Slave with Master".

There are 3 ways to make one motor spin in the opposite direction.

1) Reverse the wires to one coil.

2) Change the active low setting for the direction pin.

3) Check the "Reverse" box in Config > Homing/Limits.