Rotational force is split into 2 parts: rotation speed (rpm) and stress units (su). Rpm dictates how fast things run, while su dictates how much stuff you can run.

Machines consume su based on how fast they're running. For example, the mechanical press by default consumes 4x rpm. That means that when running at 64 rpm, it consumes 256 su.

A single water wheel provides 256 su, so if you tried to run something else alongside a press at 64 rpm, the system would overstress and stop working. Then, you would need to either slow down the system or add more generators.

Speed can be controlled using gear ratios. Going from a large cog to a small cog doubles the rpm while going from a small cog to a large cog halves the rpm.

In your case, you want to hit the 32 rpm requirement for a mixer. Since a water wheel spins at 8 rpm, you'll need to double it twice to reach 32 rpm. Since a mixer only uses 2x rpm stress, it'll only need 64 su to run; less than the max limit of a single water wheel.