It's actually not a bad thought persay and it has been suggested in the past. You can't look at Noct AST + WHM in the same sense as you would Diurnal AST + SCH or WHM + SCH.

Noct AST makes the better single target healer over WHM in some respects for three reasons now:
  1. Stronger core spells by 5%
  2. 10% less MP cost on healing spells
  3. Better MP regeneration skills

So with that in mind it's better for Noct AST to heal and WHM to DPS (and provide more MP efficient AoE heals if Medica II is sufficient) but the WHM kit doesn't have enough MP sustain to maintain good DPS in an extended fight, even with Ewer being thrown in their direction.

If you're gonna run Noct AST + WHM, it's probably better to have a more even split in healing and alternate who's DPSing to maximize DoT up time while minimizing use of the more expensive DPS spells like Stone III and Malefic II.

What you've suggested in the OP is right idea going but your group has to adjust their meta thoughts when dealing with Noct AST + WHM since the MP sustain SCH has isn't there. This is a bit of a hard sell though since it requires more coordination than normal and probably more trust versus anything + SCH combo.