Currently nothing is mapped to a second finger coming down while painting a brush stroke. I wondered, would it make sense to have that mapped to tweaking the brush size.
You can adjust the radius of the tool with 3 finger drag 
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Because 2 Finger is undo…
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It would be during painting so the second finger would come down while the first is already down. So it wouldn’t affect undo. Didn’t know about the 3 finger gesture though 
Note that you can use the sliders on the side during sculpting.
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