What about a rule of "diminishing returns". Let's make the conjecture that healing powers of any kind stimulate the bodies reserves and increase its recovery so significantly that it's instant for all intents and purposes.
Let's expand that by saying that a creature has a limited amount of reserves that are only recovered by resting.
So the first external healing (i.e. not second wind) is at full power. The second healing is at 1/2. The third healing is 1/3, the fourth etc.
So healing always works, it just works less and less as the character's body runs out of reserves to respond to the outside healing powers.
Just a random thought, it might help promote 'damage sinks' as the defenders get less results from healing that other characters move up to the front to absorb some damage.
Let's expand that by saying that a creature has a limited amount of reserves that are only recovered by resting.
So the first external healing (i.e. not second wind) is at full power. The second healing is at 1/2. The third healing is 1/3, the fourth etc.
So healing always works, it just works less and less as the character's body runs out of reserves to respond to the outside healing powers.
Just a random thought, it might help promote 'damage sinks' as the defenders get less results from healing that other characters move up to the front to absorb some damage.