Its usually the SCH responsibility to keep up stoneskin on most fights. I know WHM get the trait that enhances the % of hp , however, we (SCHs) have MANA for days and it can drain your mp if youre not careful. The only fight ive done were the WHM is keeping up ss is levi ex. OT needs that extra 8% granite skin provides. SCH will usually embrace spam the ot to avoid the debuff.
But its all pretty much summed up with the previous post. WHM is better AoE healing and SCH single target healing. In some parties ive been asked to keep up both tanks while WHM heals DPS and chips in on tank healing when necessary.