I have two rods (that will be powered via something like two water wheels), and I need to combine their power so they both rotate the same rod. Sadly, they may spin at different speeds, and the difference may be more than a little bit (and may change), so I don't think a differential as proposed by this question would work. But, I have a shaft that I'd like to rotate using these two rods. I don't really care if the power from the two rods is averaged or summed, as I can use gears to adjust the torque/speed ratio if needed.
So, given two rods spinning at two different rates, how can I combine their speed?