You can't connect multiple drivers to the same motor without risking damage.
The TI DRV8825 contains two full bridge MOSFET drivers, each of which drives one of the two phases of a bipolar stepper motor.
The motor driver attempts to keep the winding current of each phase at precisely the commanded value. In order to accomplish this, the phase currents are continously measured and the effective winding voltages are adjusted using pulse-width modulation (what you call "chopper drive") so that the currents stay close to the values commanded by the indexer logic.
PWM current control works by switching the MOSFETs on and off in such a way that the winding current is alternately ramped up and down. Current is increased by connecting the winding between the supply and ground. Current is decreased either by shorting the winding to itself leading to a rather slow current decay by voltage loss mainly over the winding resistance (slow decay mode) or by connecting the winding between the supply and ground in reverse, transfering energy from the coil back to the supply (fast decay mode). Done rapidly enough, the winding inductance evens out these rapid variations in voltage, resulting in a relatively constant current.
In theory you could connect multiple drivers to the same motor, but only if they always switch their output MOSFETs to the same state at the same time. If you cannot guarantee that, one driver might try to ramp the current down while the other tries to ramp the current up, effectively shorting the power supply to ground trough the coil driver MOSFETs.
The DRV8825 has no provisions for synchronizing the PWM outputs, so a shoot-trough is guaranteed. Thanks to their overcurrent protection the chips might (or might not) survive the abuse, but certainly the motor won't be driven satisfactorily by this arrangement.
