Reduce the output current from a stepper driver
It's possible if you have some high wattage resistors. Adjust your VMOT to give you the proper current through the 0.4V motor and rely on the internal chopper to give you the proper current in the 1.7A motor. Keep in mind that if you start chopping the output to control the current through the 1.7A motor, you will effectively decrease the applied voltage on the 0.4A motor so you'll have to get the resistances just right if you want to run both motors at their maximum current. If you match the motor current exactly, you will not be chopping at low speeds, but you may not be able to achieve high speeds with good torque because motor inductance is going to start dominating the current response and you'll have no way to control it. It would be significantly better and possibly cheaper to run with two separate drivers. High power resistors aren't cheap. You'll probably need 25W resistors to get the job done without starting a fire. If you drive the motors in series, you could use 7 Ohm resistors across the 0.4A motor windings. Then you need to drive VMOT with 15V. I don't think you'll want to try to microstep in any of these configurations.