Hi, you normally should use AST as a replacement for both healers. If there is SCH around use diurnal sect, if there is WHM around use nocturnal sect. Using nocturnal with a SCH is a vital mistake because only 1 shield applies. So go with diurnal with sch ALWAYS. But with whm, if there is no huge aoes that requires shields and such, you may go diurnal with whm too. It depends on the fight and regens stack with each other so it wouldn't be so bad. But again the ideal is to use AST as a replacement for the missing healer.