Two SCH is very do-able, but it requires a little more coordination between the two healers than any other setup.

Assign one SCH to tank shielding and healing and the other is allowed to shield everyone else. Have the scholars have different fairies out. SCH will rotate casting sacred soil on the tank, so the tank will have a constant 10% extra damage reduction. Tank healer spends most of his time casting adloquium and physick. The other SCH cast physick on the tank and spot heals and succor on everyone else. Honestly, with two people procing succors from sacred soil and two fairies, the party shouldn't require much spot healing.

Now two whm or sch and whm, no preplanning or coordination is really needed. If both healers react to something, the worse that happens is some over healing. Thus why the double sch is least desirable.