Perhaps in the affect of healing you can only heal what you know.  And while they only use one or the other side of the power the ability to use the power is from a "weave" of both.  On that assumption when you still or gentle all they have been doing is severing half of the "weave" allowing them to access the power.  So in order to truly be removed from the power you would have to stilled/Gentled with both sides of the power.  This would fit the balance principle in that neither side would have absolute power on the finality of a cut from the source.  Forcing both side to work together to cut and heal.   

 

That was exactly what i was trying to say :D I'm glad someone could find the words :D

To sum up, and to paraphrse Bernard Black;

"It is the way it is because that's the way it is. If it wasn't the way it is then it wouldn'd be the way is".