Eric used to have a Groovy smartapp called ‘Shady’ that was specifically for this purpose. It would group shades into one virtual shade who’s state was basically an average of the position of the shades in the group.
If I remember correctly the code was not very complicated at all - and would probably work well in LUA. maybe someone who’s into driver development like Mariano would want to try porting it to LUA as a project. I think custom presentations have evolved to a point this might be possible?