With a suitable voltage from the power supply I would not expect any loss of torque. Stepper motor torque falls off with speed due to the inductance of the coils and back emf. And of course two motors in series doubles the coil resistance as well as other things. A higher voltage counteracts that. Of course it is essential to set the driver current limit correctly to protect the motor. If the motors are in series the current will be the same as for a single motor.
...R