Se puede pero no con esa librería. Debes usar ésta librería AccelStepper Library
Destaco esto de la librería
You can create multiple AccelStepper objects, giving a unique name to each motor. AccelStepper can manage any number of motors, as long as you repetitively call their "run" functions.
Pruebala y nos cuentas.
AccelStepper significantly improves on the standard Arduino Stepper library in several ways:
Supports acceleration and deceleration
Supports multiple simultaneous steppers, with independent concurrent stepping on each stepper
API functions never delay() or block
Supports 2, 3 and 4 wire steppers, plus 3 and 4 wire half steppers.
Supports alternate stepping functions to enable support of AFMotor (GitHub - adafruit/Adafruit-Motor-Shield-library: Adafruit Motor shield V1 firmware with basic Microstepping support. Works with all Arduinos and the Mega)
Supports stepper drivers such as the Sparkfun EasyDriver (based on 3967 driver chip)
Very slow speeds are supported
Extensive API
Subclass support