Organ Voices on the ES default to SPEED CONTROL on MW.
The Mod Wheel always sends cc001
You can select which physical controller is responsible for changing the Rotary Speaker Effect's "Speed Contrl" parameter from either Slow-to-Fast or from Fast-to-Slow by setting this in the Voice' CONTROL SET
Pree [EDIT]
Press [COMMON]
Press [F4] CTL SET (Control Set)

If you set this to Foot Switch and that foot switch is a 'momentary' type pedal, you will need to keep your foot on the pedal to keep the speed at the opposite setting. The TRANSITION TIME is programmable with the Rotary Speaker Effect itself.

You say by default they are on the MW but can't I then assign control over the MW to a foot controller? I have researched/read some old Motifactor posts on topic (attached a clip below). A lot of the debate seems to be about freeing up your hands to flip switches; however, for me its more than just on/off, its more about the time it takes to find the sweet spot (desired rotation emulation) on the MW to wind up or wind down. I guess it used to be a switch on/off on older Motifs, now its variable speed on ES....I find myself spending too much time during a live performance rolling the MW. Was thinking my left foot was a better controller option for this effect. Second question- I know ES has two foot controller inputs, First, I was considering to use two FC7s for MW and Volume control. As an alternative- is it possible to assign both MW and Volume to a single FC7 foot controller? I understand, musically they might now work hand in hand, but maybe for some voices, it would.....if its even possible to have one foot controller assigned to do two things at once?

A lot of the debate seems to be about freeing up your hands to flip switches; however, for me its more than just on/off, its more about the time it takes to find the sweet spot (desired rotation emulation) on the MW to wind up or wind down.

First, let's shed some light and clear up a common misconception, especially since you think there is a "sweet spot", there isn't, really. That's not how this works!!!

The MW does not control the actual speed of the rotation as you are thinking. The MW is acting as a Switch. And like a light switch it controls two conditions. For the light switch it is either ON or OFF. For the Rotary Speaker effect it is either SLOW or FAST. The MW sends values 0-127... If, for example, the Organ Voice starts with Slow selected, and you set so the DEPTH = +32 (linear response) all values 0-63 = SLOW, and all values 64-127 = FAST. You are simply flipping a switch.

The actual speed of rotation (in cycles per second) is handled within the parameters of the effect.
That is, within the effect you set what speed is slow in cycles per second, what speed setting is fast in cycles per second
And you set how long it takes to transition from slow to fast, once the "switch" is flipped. Slow-Fast Transition Time, and Fast-Slow Transition Time.

Background:
On the "real thing" -I'm talking a real rotating speaker, the slow ramping up and slowing down is a result or wear and tear. It is not how a proper Leslie worked. Only when the convey belts got older and started slipping did this happen. A brand new Leslie would jump from one speed to the other in the blink of an eye. Why the ramp up and down became popular is urban legend, but back in the day, folks used to replace the belts when that got that loose.

It became 'hip' at some point to have loose belts (go figure). It's one of those things that was not intended but became popular, stylish, 'hip'... Not intended. ((Did you know the Clavinet was designed for classical music... It's use as a funk rhythm keyboard was totally unintended!!!))

So in emulating this behavior the parameter Transition Time is programmable. There is a separate Transition Time for the high frequency Horn, and one for the low frequency Rotor... In general the Horn belt wore out differently than the big Rotor belt... If you explore the Insert Effect and experiment you will discover how this works,

And you will discover that the MW is indeed just a Switch with two positions, the Transition Time is a totally separate programmable parameter. All you can affect with the mod wheel is Slow or Fast... How long that transition takes is a totally separate parameter, period. If you play around with it you STOP a thinking there is a sweet spot, there is not, you're simply flipping a switch, the "belts" take over from there.

Although written for the Motif XF, the effect and parameters are the same: please read this...
The Rotary Speaker Effect

Let us know when the light comes on! 🙂

OK...in other words...Motif does it just like the real thing, baby.........I push the MW up or down button far enough to flip the switch to start or stop the belts,spinning the horns and rotor speakers fast or slow or at whatever two volumes, speeds and transition times I want to insert. No limits.....If I want to flip the switch with my foot, I can assign a foot switch and then can press and hold a momentary type pedal OR assign a foot controller and seesaw on a volume pedal far enough to flip the switch......

Bingo!! 🙂