mmm...since you mentioned it, im confused as well, but what I understand of it, is that crit heal < 2*normal heal. Let's say your normal heal is 500, if you cast normal heal, you will get 500 heal+ 500 dmg shield, however, if you crit heal, you'll prolly get around 750 heal + 1000 shield. I believe that is the reason of the tooltip purposely mention about the double amount of the shield.

Does the shield requires hp to be actually restored or it include overheal? just something I had debated with others I game.