How to route a stepper motor through an H-bridge and back into an Arduino?
Ref: http://www.orientalmotor.com/stepper-motors/technology/
You have a 6 lead motor designed for unipolar Nch switches, yet you are using a bipolar half-bridge IC. (BTS 7960 is not an H bridge.)
Change drivers to a dual half-bridge with V+ on both centre taps
or change the motor wiring to a 4 lead bipolar and H bridge driver for each coil.