It's not just a matter of matching what's stated on the tech data sheets... but also a matter of meeting the actual peak demands of the motors when you have wired them to suit your needs.. you have to confirm that what you think is happening is what is actually happening...

Using your data as an example. Most specs are per "active" phase. that means what the motor will require with a single phase active. You state that your motors are 2 phase... however this means nothing really.... internally they might well be 4 phase motors pre-wired to run as 2 phase but with 2 actual phases active per step... this would double your current requirements at peak load if it were the case, unless the supplier has worked this out for you already and supplied this data as the rating.

I would guess that they are bipolar parallel wired internally and that the stated load of 4.2 amps is actually for 2 active phases... but this is just a guess as it common for "2 phase" motors to be supplied in this configuration.. it gets twice the torque out of the motor so manufacturers tend to do the wiring for you internally and you have less to worry about.... but you do need to confirm this is the case before spending the pennies buying something to match.

If you have this motor on all 3 axes then you would be looking for something that can output 24 - 42 v at a maximum current of 10 - 12 Amps.... generally speaking you will only ever draw about 2 thirds of the motor peak current load. It's not necessary to give the motors every volt they can use as it's doubtful that you will ever be running them flat out. Motor speed will be more governed by the acceleration and deceleration loads that you machine structure can handle than what the actual motors are capable of.

Generally speaking Voltage = Speed and Current = Power much more important to get current load sorted as this effects your torque ... most of the time you will probably be in the lower part of motor speed range but will want maximum torque... steppers supply most torque in the lower speed ranges anyway.

hope this helps....