You seem to be operating under the assumption that most content in this game has somewhat harsh HPS-requirements throughout the fights.
Unlike some other games wherein tanks take a more consistent beating, in this game the fluff-damage is mostly negligible (exceptions exist - no need to call me out on that - speaking about the bigger picture). You have the fluff-damage and then the scripted spike damage. Tank killing moves at certain intervals, followed by more fluff-damage. Repeat. That is the norm for this game.
Now, while in some cases there might be other damage being inflicted at the other tank or the party - meaning that two healers need to actively heal - it is very common for there to be times wherein it suffices that only one of the healers is "maintenance healing".
By maintenance healing I refer to healing which is not a MP-strain on a healer. That would translate to keeping Regen up, Cures and then Freecure procs as necessary for a WHM. For the SCH, that would more or less mean microing Embraces while DPSing. (Or in some cases, add some manual self-casted heals in the mix and then resuming DPSing).
If we can achieve a comfortable level of maintenance healing whilst dealing some damage - we have struck gold. At times, you can extend this to calculated risk-taking in resource usage, ie. knowing that even a moderate MP-strain from having one healer overwork a bit will not result in exterior support being any more necessary than it would be otherwise. (Namely, not needing a Ballad due to this kind of careless play of the other healer).