When the motors are spinning, the average current is always going to be much less. The coil inductance limits how fast you can spin the motor and current draw. The most current a stepper motors uses is when it is at zero rpm. That is the current requirement for max holding torque. It also depends on what step position in the cycle the motor position is at. At one point in the cycle both windings are fully energized. That is the position when you need to take your current readings for maximum draw. Power supply needs to be able to handle that for the number of motors connected. Else maximum holding torque will not be achieved.