Why they wouldn't run in sync in parallel is very confusing, I don't see how they could actually run out of sync. But apparently they can.

See post #5 and others.

Sure you dont havr the motor set back wards I had the same problem when I first turned my machine on. All you need to do is switch around two of the wires so the motors run the other way. Like i said the motors hooked up in pareal will not work. dont ask as I dont know why. Probly a big forumla. If you leave the motor in it will work for now. But it will give problems later on. Just be prepared to remove them later. nice work.

I would think it's because I am using bipolar steppers. As they only have 1 coil, maybe the current in the coil that is turned on, is somehow being transferred to the off coil in the other motor, creating drag.

No it doesnt matter what stepper motors you use. they do not  use one coil the all use 4 wire basic some have a center tap and so on.  but they all have 2 coils reverse 2 wires and the motor run the other way check on the internet for stepper motor basics  also see it you can find the spec sheets on your motor. It will tell you the power amp and voltage  It will also tell the tourque at different sppeds. Tchnicaly they have more than two coils but run on the principle of two coils The stepper motor basics site will also explain why you canthook 2 motors in series or parrel. To much math for me ...

Hi did you get a chance to check out the gecko drive on the internet they have a motor controler that is called g540 type that in then go the the g540 control box check out stepper motor basics good reading and you will get a lot of answers about you stepper problems. It also gives wiring and power supply information.