Why have you added a DC supply between A and B to the steppers?
The whole schematic seems wrong. I cannot see any paths the current through the stepper can go from positive voltage supply to ground.
All you need is eight transistor. All emitters grounded, aresistor on their base and connect all stepper wires to each transistors collector. You might choose to use the darlington.
Further on, running 12V on a stepper on low speed will probably cause the stepper to overheat. You then might consider getting a couple of PNP transistors so you can switch between heavy duty mode for high torque and high rpm (12V) and economy mode (5V) for slow rpm and less torq.