For content that is above your gearing level, two heals is an advantage provided there is no enrage timer.
The mana efficiency of only having to cast every other heal and having both healers regen mana at once.
Plus the bonus of having two heals at once, if you really need it.

That said, for one-group content...
You'll generally not need more than a single healer at a time.

And once you out-gear the content, you'll frequently not even need a dedicated healer, provided your DPS can heal in their DPS forms/specs.



My top PvE team was:
- Tank: Prot Paladin
- DPS: Destro Warlock
- DPS: Elemental Shaman
- DPS: Elemental Shaman
- DPS/Heals: Elemental/Resto Shaman

For most content, 3x Elemental was sufficient heals.
For harder stuff, one of them could spec Resto... and if things were really above my level, two could have gone Resto.
A Priest would have brought a better buff, than a Resto Shaman (given 2 Elemental Shaman already on the team), but 3x Shaman was both simple and easy.
A Druid for heals would have been an option too, and Balance had the same spell damage buff that Warlocks brought... but Warlocks just needed to apply a curse, while Druids needed to actually DPS... with their more complicated rotation; plus if the Druid went Resto, that would remove the 13% spell damage buff.



I'd probably go:
- 1x Tank.
- 2x Pure DPS.
- 2x Caster DPS with Heal Specs.

That could overlap, some.
But give yourself the option of healing when you need it.
And give yourself the option of additional DPS, when you don't need the heals.




Your DPS is mostly going to be one or two button spam.
You might have one key for AoE and another for Single-Target stuff.

You'll have a hotkey for buffing.
And a hotkey for a few important specials.

Maybe hotkeys for crowd control.
- Warlock Banish/Fear/Seduce
- Priest Mind Control/Shackle
- Mage Sheep
(Probably set up an FTL assist system, then use /focus (and raid marker symbols) and /cast [@focus] Polymorph type things)

There won't be a lot to manage.
A single-target rotation is going to be a single target rotation, whatever the spell sequence.
The mage will receive, 1, 1, 1, 1, 1, 1, 1, 1, 1; as will the Warlock, S.Priest, and Druid (when you spec Balance).
It doesn't matter if the mage has (on hotkey 1):
/castsequence Arcane Blast, Arcane Blast, Arcane Blast, Arcane Missiles
While the Warlock has (also on hotkey 1):
/castsequence Immolation, Incinerate, Incinerate, Incinerate...

Having the option of a Disc Priest, Holy Priest or Resto Druid...
That's flexibility.
You have the option of 2x DPS, 2x Heals or 1x DPS & 1x Heals -- whatever works best for your current situation.