^
Don't forget that it is not rare at all to see the case where one healer is targeted to conflag. and many of time same person is targeted twice.
Mr A. used divine sell : (first time 2 stay, but A plan to use it first.)
Mr. B enter the conflag, leaving A alone. (so it remain A alone without Divine, he would need to use benedict of source. (or presence of mind + cure 2 spam, but he will end up no mp.)
Mr B enter conflag, again leaving A alone.. now A either run out of mp or have no emergency skill already.
Not to mention, another ground breaking part is that when the tank doesn't have defend ability, and no adlo from SCH then he is usually 1-shot. stoneskin is usually slow, and in case of one healer get into conflag, you have no time to do so. Therefore, a lot more problem compare to SCH who can manage all this very easy. (so this mean they are not equal heal for single target.) As I said that it is not impossible, but it is very hard and I bet that there will be a lot of issue/mistake despite following your method.
in SCH case, he has 6x lustrate and fairy, he can solo heal while the whm enter 2-3 conflag easily, I have seen it to even some new sch who doesn't play very long too. Why do you think every content so far have party recruitment of 2x healers, but then only Turn-5 that they specific 1 SCH 1 WHM ? It is common understanding to them that SCH and WHM are different. WHM is good at AoE, SCH is good at single heal.
Whm can cast regen into multiple person, but SCH can't call multiple fairy to embrace multiple person.
Whm can cast cure III, big AoE heal from distant that SCH can't.
Whm medica II can be used more often than SCH fairy AoE combo.
Then,..
SCH can heal tank to more than the max HP he has. SCH has double high potency when critical that Whm doesn't.
SCH can use emergency skill for 3-6 times in a short period, while WHM can only use once and WITH HUGE DELAY (working as intend wtf).
as OP explain, single target potency of fairy + SCH combine is always higher output than whm.
So people should admit that they are not equal. they strong on each different role.