Its interesting about the completely opposite responses about how the
Half Duo-Art expression levers work on various pianos.  On my Aeolian
upright piano it has a single metal plate with two levers labeled Bass
and Treble protruding through one slot.  The plate is also labeled with
soft to the right and loud to the left.

The springs have never been modified and they hold both levers in the
Soft position.  If I remember properly, the slide valves in the regulator
are fairly small and can be set to either the left or the right side of
the exhaust hole going to the foot pump.  If this is the case, you should
be able to make the levers work in either mode, normally Loud or normally
Soft, based on how the leather nuts are adjusted on the regulator.

Pete Knobloch
Tempe, Arizona