You appear to be in luck!
The pots are ground referenced, via a resistor at the "low" end.
Terminal 2, where you have the negative of the 24V supply is also signal ground.
The "input point" looks to be the two joints between the pots, where the cathodes of the two diodes connect.
It appears to select which pot is in use by feeding power to the top of one or the other, so only one passes a voltage through those diodes.
Turn both presets to minimum speed and you should be able to feed an external voltage to that point to set a speed.
The voltage required will be trial and error - it could be 10V for full speed or it may only be 3V for full speed; you will need to experiment with that.