Most players play with two straps and the right hand is anchored. So the piano keyboard stays fairly still under your chin while the left hand (and shoulder) work the bellows.

The left hand fingers are often out of sight unlike the right hand ones.

The stradella keyboard is ingenious and from the left hand edge provides counter bass, fundamental bass, major chord, and minor chord on a 48 button system. It provides much more flexibility and ease of use than a second piano keyboard on the left hand side.