Increased Torque

If the motors (and their applicable drive ratios, if any) have the same output rpm, in theory you could simply add all their output torques to get the total torque, and add all their hp (or kw) to get the total power. However, if the motors differ in these respects, the faster ones will tend to drive the slower ones, and the total torque and hp/kw will be less than the theoretical.

It is done all the time in large installations, and ocean going ships.

However the Motors must be of equal performance, and their operation synchronised.

Basically you get the sum of horsepower/torque less the losses incurred by the transmission.

The PAD Ro/Ro ships had 3 18-cyl, Pielstick diesel 19336 kW motors connected through a summing gearbox driving a single variable pitch screw(propellor).

Depending on the operation they could run 1,2 or all three motors or combinations of to suit the conditions.

Ref; MV Cape Edmont (AKR-5069)

Note the listing says one motor there are 3.

It is the characteristics of the load that determine the power required from the motor(s).

ref: http://www.daerospace.com/MechanicalSystems/GearsGen.php

Done all the time on large rock crushers, mainly because if you need 800HP, it takes a lot more machinery to move and mount an 800HP motor compared to 2 x 400HP motors. It also helps the bearings and belts by allowing balanced forces.

Here's one I worked on a lot:

This kind of gives you an idea of the twin motor concept, coupled to a common shaft: