I am not sure whether you can use dual servo drive unless the loop is closed in the controller such as Galil etc, where you can use electronic gearing.
one motor is slaved off the masters encoder, although the slave does still have an encoder also, with this system you can even change the 'gear ratio' to whatever you wish .
Systems that use steppers can do it because the motors are synched by the identical stepper pulses.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.