2 motors on X how do I setup Mach3

[i2i]

That's one way - you can also connect 2 stepper drives to the same mach3 output or if the drive is powerful enough even 2 motors to the same drive.

I wouldn't put two motors on one driver.

Don't do that both are bad idea.!! If 3 axis then you have spare outputs so use them, it will far more reliable and accurate.

I don't see that using the c axis and the x axis would be more reliable or accurate than using the port output to supply two drives. Have you experienced problems doing this.?