Using a DC-DC boost converter to run a stepper motror through a chopper drive
The stepper motor will act as an alternator when spinning and generate a voltage opposite to the drive voltage proportional to the angular velocity. To counter this voltage known as back-electromotive force (back-EMF) and keep the torque, which is proportional to the current, high, the driver ramps up the voltage as the motor speed increases. To allow for this voltage headroom, the motor is designed to have a winding resistance lower than what would allow only the nominal current (e.g. 1A) to flow at rest when given 24 V.

The pololu driver is fairly autonomous and actually has current feedback to counter this: it can adjust the motor voltage (via pulse-width modulation) in order to keep the current within limits at any speed. The board will function with motor currents up to 2 A per phase, so 4 A total.  

If you set the driver to its maximum current of 2 A per phase (as you stated) and the winding resistance is 12 ohms or less, it will draw 4 A at 24 V DC (96 W!) in the worst case. Powered from a 6.7 V (7.4 V - BJT forward voltage) source, the boost converter low side switch would have to conduct a peak current of 4*(24/6.7)*2 = 28.7 A. The low side transistor of the LM2588 boost converter IC has an absolute maximum current rating of just 5A, so the 24 V will most likely brown out immediately when the motor is driven.

You can set the current setpoint by adjusting the little trimmer pot on the PCB. You should set the current limit to 400 mA per phase or less, or you will operate the boost converter out of specification.