Rtm explained how to do this on the Beta Forum somewhere. IIRC you need you set your timer's input to Speed, and then do a sum of all the possible healing inputs and subtract from that the sum of all possible damage inputs. This way, when there's two damage sources, their values will be summed and result in a negative speed on the timer, causing it to drop faster. Likewise, a healing source will oppose a damage source, etc.

