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.