EDIT: As Bruce Abbott pointed out in his answer, it's not possible to connect 3-wire brushless motors in series, since you don't have access to both ends of each winding. Oops. So, the more relevant question is whether there are any potential problems with wiring multiple motors in parallel, other than the increased current draw.
ORIGINAL QUESTION: I'm working on a design that requires a large amount of torque in a very confined space. Due to this and other requirements, my best options seems to be to use two motors to drive a single load. The shafts would be connected together, so the two motors would always maintain the same relative orientation.
I would like to avoid using two motor controllers, so I'm thinking of wiring each phase of the two motors in series (i.e. phase 1 of motor A is in series with phase 1 of motor B, etc.). I would have a single encoder on one of the motors, which I would be using for position feedback and for sinusoidal commutation. I'm expecting that this would effectively allow the two motors to act as a single larger motor, with double the voltage required for a given speed. Is there any reason this wouldn't work, or any potential issues I need to watch out for?
One thing I'm worried about is that I would need to make sure that the connection between the two motor shafts is "clocked" correctly such that the rotors of both motors have the same position relative to the stator windings. There will inevitably be some error in this; I assume that this would just slightly reduce the torque for a given amount of current, but perhaps it would cause other issues?
Some additional information, in case it affects the answer:
This is for a positioning application which uses a haptic operator interface. This uses the motor current to calculate the motor torque, which is then used to provide force feedback to the operator. Therefore, one of the main requirements is very low torque ripple.
The motor speed will be very low (from 0-100 RPM).
The motors I would be using are the EC-4pole motors from Maxon.
I will likely be using the EPOS4 motor controllers from Maxon.