I'd enjoy trying it out but SCH + Noct AST would really not be easy because of the override and the lack of options for picking the party up. If AST had one half-decent burst AoE that wasn't shield-oriented, it wouldn't be so bad, but AST already struggles with AoE and it's not SCHs forte either, so taking into consideration that AST shields would be completely useless in an AoE event, it wouldn't be ideal. SCH would have to shield everyone and then both jobs would have to fight to pick everyone up. For single target(s) I could see it maybe working, but AoE would be a horrible experience.

As for WHM/Diurnal AST, that's already common practice for a lot of ASTs, since Regens do stack.