Was about to get a toy- then I did some research...

[AndyUK]

Or is this 2/3-rule applied simply due to the fact that the motors are seldom simultaneously drawing all of their rated current?

I think its that you'd never power all phases of the same motor to 100% of their current at the same time? But I'm sure one of the electronics experts around here will come up with a much more detailed answer! The 2/3rds rule is for parallel wiring, and for series wiring you can get away with 1/3rd.

I'd wager the answer is in here: http://homepage.divms.uiowa.edu/~jones/step/index.html (I just don't have the time to read it all!)