In a internal combustion engine, for example, we know only one piston is fired at one time, no matter on the number of cylinders in the engine: 4, 6, 8, 12, etc. Why cannot two pistons be fired at the same time?
In fact, linear forces can be added: for example, when two or more people push a stuck car, by adding engines to an airplane.
In a conveyor belt there cannot be 2 electric motors.
Why torque cannot be added whereas linear forces do?
Torques can be added.
Imagine a large horizontal wheel that turns a shaft. Now if one man applies a force to turn the wheel, the net torque will be some value. If two or three men join in and apply forces in the same direction (clockwise/anti-clockwise) then the net torque will increase.
This is commonly applied in machines such as human powered capstans...
This is fine if the machines have a low RPM such as merry-go-rounds and capstan shafts. But in high RPM machines, the complication to having multiple torques acting at the same time is that they need to be synced perfectly and they need to act at the exact same instant in time. Small variations in timing could result in large vibrations on the shaft.
In high RPM machine aircraft turbines, the torque added by each blade is not "timed", instead, the torque acts continuously as the blade revolves around the shaft. So multiple blades around the periphery of a shaft act in perfect unison to generate a large combined torque.
It's not a matter of "can" it's a matter of "should." If every cylinder fires at a different time, the torque is smoother because it is evenly distributed. If you had multiple cylinders firing at the same time, you'd have a period of time with much more torque, and then a longer region with no torque.
