When you have two joint controlled morphs for a body part, is it possible to have one of the morphs influence the geometry only if the joint moves in positive values, and the other morphs affect the geometry when the slider moves in negative values?

Try this, set up two channels. In your positive channel set forceLimits to 4 and set your min to zero and your max to the required upper value. In your negative channel do the reverse.

Ahaa, perfect! Thank you PhilC.