Hahaha! But would it be possible? I mean, I never tried to connect a second mouse, I don't know if Windows supports that. I mean, looking at my touch pad, the only outcome is that the input of touchpad and USB mouse are combined...
(I remember that it was possible to connect a serial mouse and play Doom Legacy on the same computer with a split screen...

)
Anyway, doesn't matter, no real option.
I don't know if it is of any interest, but I wanted to say that I had a second thought about my suggestion "copy (movement) from target". Instead of two possible options, namely "copy from target" and "inverse copy from target", it could be unified into one Option by freely setting the direction into which the first (major) mouse movement should be converted. The actions would be:
1. set target (e.g. right breast)
2. set direction (e.g. "East" when looking on a compass)
3. grab second target (e.g. left breast) and move (e.g. "West" on a compass)
Through this set of actions, I think that the movements on the horizontal axis are inverted, which would make it possible to move both breasts in a counter-rotating movement.