If the AST had lower potency heals than SCHs and WHMs, they'd struggle in Light Parties where they were the sole healer.
SCH works out fine because Adloq = Cure II when accounting for the Shield and Eos is free spam healing.
The RDM didn't replace the BLM and WHM nor sword users because those Jobs brought something to teh table that the RDM didn't, hence why it supplemented them rather than outright replaced them.
Do you really think the BLMs would get a place in XI parties if RDMs had the exact same offensive power in their spells while also providing healing and buffs? I think not.
And yet WHM+WHM is still allowed to stack.
The issue wouldn't be with the stacking of shields, its the ability to provide both shielding AND big heals at the same time.
It'd be like taking 2 WHMs and getting a free 9th slot SCH in your raid team.
You would have the ability to provide 2x large heals in addition to being able to suppress damage via shields when required.
If both ASTs stayed in WHM mode with one of them designated to switch to Barrier before any major AoEs to prep shields and then goes back to WHM Mode to provide those bursty heals, you essentially get 3 healers worth of healing in.
