Not sure if this was said or not yet, but whm and sch healing are quite different.

WHM healing is focused in actually -healing- taken damage, where sch healing is more about mitigating said incoming damage and topping it back off

only shielding whm gets is stoneskin (18% max hp of target), and a defensive buff (protect; physical and magic defense)

sch has Galvanize and access to stoneskin (10% max hp when used by sch), sch can also use protect, but they don't give the magic defense boost with it.

As Galvanize, say I heal you with my adloquiem for 1000 hp, you get a shield that blocks 1000 damage, if that crits, that's doubled. I think the best adlo crit I had in T5 (20% echo buff+ full party buff+Eos' healing buff active) was 1780, that gives a 3560 point shield in which the tank took no damage, allowing the whm ample time to get the entire group healed up effortlessly. There is a level of synergy between a good sch and whm, same can be said between DPS and Tank classes when they -work together-