If you run two motors on one drive you cannot take it above about 2 RPS or it will break out into violent oscillation because the motors must then be kept in their constant torque range. If two motors are being run from one drive they will draw a current that will vary with the load. This means that one motor would use up all the current and leave the other motor to stall out if it is taken above that speed. Step motors do not play well with each other. Consequently, one drive is meant for one motor.
What kind of motors do you have on all of your axes?
If you are unsatisfied with the G540 we have a 15 day no questions asked return policy, and we will gladly refund your drive and shipping costs if purchased directly from us.