I don't think you can, unless someone chimes in otherwise.
If you use one pot there may be interferance that would upset the control of the controllers so it would be better with two pots shafted together to control two controllers. You would need some fine tuning to get them to both power up syncronously. That would require a matched pair of pots and some fine adjustment for max and min stops on each one.
You couldn't 'series' (or parallel) the outputs from the controllers as the PWM wouldn't be in sync.
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