Neat find. I'm having trouble trying to figure out how that thing was used as a drill guide - looks like it was engineered to be a linear bearing housing.
From what I've read, it's just a software setting in Mach3 to slave two axis together so any signal sent to motor 'Y' is also sent to motor 'A'. Some people use dual shaft motors and put a timing belt between the slaved motors as an extra insurance policy to prevent racking the carriage if one motor should miss a step. Read through the looong JoeChevy 4x4 hybrid thread - there is a brief discussion on using double flange bearing supports to eliminate whipping on the 4' leadscrew with rapids at 300ipm.