That's one of the rare times in a raid when you spam Shadow Mend. It's an excellent spell to use for anyone taking constant, especially moderate to high constant, damage. So if *only* the main tank and the DPS-standing-in-bad are taking substantial damage, you just spam them individually with Shadow Mend, probably focusing more on the DPS since the other healers are more likely to be taking care of the tank. Forget your regular atonement rotation until the DPS gets his act together or you exasperatedly Leap of Faith him. Even without Grace, which you're unlikely to select in a raid, Shadow Mend does both good HPS and HPM in that situation.