The drivers used for 3D printers are constant current ones, and the motors are normally chosen to have a voltage drop of no more than 4V at maximum current. So you can connect 2 motors in series without any loss of current in the steady state, using a standard 12V supply.
However, when the motor is moving, additional voltage is needed to overcome the inductance of the winding and to overcome back emf resulting from the movement. The higher the speed, the higher the voltage needed. So with 2 motors in series, the torque will drop off at a lower speed than with just one motor. Whether this matters depends on what speed you are aiming for. Using a higher supply voltage (if your printer electronics allows it) will allow a higher speed.
However, when the motor is moving, additional voltage is needed to overcome the inductance of the winding and to overcome back emf resulting from the movement. The higher the speed, the higher the voltage needed. So with 2 motors in series, the torque will drop off at a lower speed than with just one motor. Whether this matters depends on what speed you are aiming for. Using a higher supply voltage (if your printer electronics allows it) will allow a higher speed.