would this be OP?

[HyoMinPark]

On the other hand, it's both healer's responsibility to keep everyone, including the main tank, alive. You do feel the difference of not having two regens on a tank in Savage and Ultimate.

Even in Savage and Ultimate, you do not need to stack double regens, be it on the party or on a tank—Medica II + Di Aspected Helios is overkill; Whispering Dawn + CU is overkill; Whispering + Di Aspected Helios + CU is definitely overkill (our AST/SCH actually did this for Garuda cleanses during prog and the overheal was 50~60%). It just isn’t necessary. Speaking as someone who has done the content, double regens end up being an overheal more times than not, and any time you’re overhealing means you’re wasting your resources that could be better used elsewhere.

There are high mobility/AoE phases that you don't have the time to babysit the tank's HP while keeping everyone else alive that they don't want to move into your range for.

If people aren’t moving in for heals, that’s on them. Not on you. And it doesn’t mean Noct AST suddenly needs more tools to make dealing with that easier.

For high mobility situations, you either master slidecasting, or you make use of other tools at your disposal (Swiftcast or Lightspeed—the latter of which has made AST better than WHM for fights where you have to do a lot of moving, such as Final Omega).

SCH's Fairies both contribute to this passively at no MP or GCD cost but doesn't get the benefit of getting to regen multiple targets. WHM and Diurnal AST both have ways to do this via their GCD and can multi-regen both tanks in two-boss fights. Whispering Dawn also exists as an AoE regen on a far lower cooldown than Collective Unconscious and Asylum at a strong enough potency to match a WHM and Diurnal AST using Medica II and Diurnal Helios respectively.

Again: double (and triple) regens end up being more of an overheal than not. There is no content that requires you to stack multiple regens to keep the party (or the main tank) alive. Not even Savage or Ultimate, and especially not casual content.

If you want to make up for the auto damage, you basically have to sacrifice more GCDs than a WHM would to make up for the disparity in Noct Sect. GCDs you sometimes don't have.

If you’re a Noct AST, you don’t spam shields to mitigate autos. You pre-shield for tankbusters and AOE raid damage, but you do not spam them. They just aren’t MP efficient. The same goes for SCH and Adlo/Succor. Auto damage should be mitigated by a Regen, which, if you’re paired with a WHM, it is their responsibility to apply. Not yours. If you aren’t paired with a WHM, then you should be in Diurnal Sect yourself, not Noct.

Playing Noct AST in duty finder when the WHM doesn't regen is absolutely awful for this reason. It's worse in Savage because you and the tanks have to make up for it.

And it’s the WHM’s responsibility to regen. If they aren’t doing it, then that is not the fault of a Noct AST. Someone playing a job incorrectly doesn’t mean you need to give another job tools to make up for someone not playing their own job correctly. Giving AST more tools like the ones that have been suggested in this thread will just make it far more valuable in content than WHM, which is already struggling against AST enough as it is (and who has no hope of competition against SCH).

Personally, I call out WHMs that I don’t see using Regen when I’m in Noct Sect. I do not think I need a passive regen a la a SCH fairy to make the healing in Noct easier.