I'm not familiar with stepper-motor driver ICs but assume they have H-bridge outputs which switch more or less rail-to-rail? If so, then it should be possible to tap off from their outputs suitable voltages to drive, via series resistors, the bases of at least 5A-capable bipolar transistors working with a ~2V supply. As for the ~2V supply, using LM317s in parallel is not a good idea. Better to find a 4A-capable (or better) chip. I don't know of one off-hand, but Googling should hopefully come up with something suitable.
I've been assuming you need a ~2V supply, but IIRC from another thread on this forum stepper-motors are usually pulsed with a higher voltage initially, to get them moving, and then the voltage is reduced. You will need to research this subject before deciding on the power supply required.