i think the answer is here in Configuration_adv:
// Dual X Steppers
// Uncomment this option to drive two X axis motors.
// The next unused E driver will be assigned to the second X stepper.
#define X_DUAL_STEPPER_DRIVERS
// Dual X Steppers
// Uncomment this option to drive two X axis motors.
// The next unused E driver will be assigned to the second X stepper.
#define X_DUAL_STEPPER_DRIVERS