lol Double SCH will always outshine Double WHM, but imo most of the time WHM/SCH is usually better.

When I decide to SCH/SCH its usually with a friend and we split party between us, when we know AOE dmg is coming we just aldo everyone, which is fine imo.
Communicate!!, if its a sequence of AOE damage e.g timults, decide who succors 1st, then when the other SCH is half way in cast start yours, that way you roughly interlock and most of the damage is dealt with, im sorry, dont use laziness as a reason to ask for a buff.