One Axis Driven by Two Motors

Why would it not be? If the amplifier can produce sufficient power without distortion.

Does your head hurt often??

No doubt it depends on who is trying to accomplish the control....

That depends on the servo loop. DC motor, AC motor, electrically commutated? Is the loop a torque command, velocity command, position command. Is there an inner loop controlling the motor shaft for one of the above, and an outlerloop that controls the desired arm position?

Can you post a picture of what you mean?

Is that amplifier the reason for the insufficient torque? Would another amplifier do?

Can you slave one actuator to the second one?

Did you look at the R2D2 plans?

I think precision is possible but needs a better understanding of what you have already.

Yes, this can be done. For obvious reasons it is easier to do this wrong. This will probably be less reliable because more parts can fail. None the less, this can be done.

How big is your robot arm? Or what is torque expected to handle?

SSS clutches with similar functions are being used in process equipment.

yes. Imagine one person pushing a broken car. A second person and third can join in and help. What isn't so easy is ensuring each of the guys is pushing with the same amount of effort. So it is with your motors. You may get a 40% + 60% split between the effort. You might get lucky and get to 50%.

ya can be used parallel .if the amplifier current is the sum of both motors.but the ref encoder will remain on the one motor .also you can use multiple drives with single motion controller by parallel pulse/direction signal.

Possible it is, probable maybe not so.

When you have two actuators acting on a single movement direction, the actuators need to be very well synchronized otherwise they may counter each one.

OK, instead of buying another motor, why not just buy one with more power to replace the one you have??