The main reason Dedicated healer/Support healer split would not work is super easy to foresee: in the format this game has for encounter design, healing is binary. You have enough, or you don't and wipe. Coincidentally, this is the reason the devs don't 'understand what we want' from healers. Tanks don't complain that they 'want to do less damage, and focus more on mitigating and holding aggro', because everyone playing tanks likes to hit Confiteor and Double Down (well, when they crit). Only healer seems to have this split of players, where some want to do more damage, and some don't want to. Anyway, back to the point, once the 'Dedicated Healer' is at the point where their healing is causing big amounts of overheal, the questions come up: Do you force them to swap to a Support healer for more damage? Do you ask them to do damage when they don't need to heal (completely negating the point of them being 'the dedicated healer')? Is there some reward in place for them overhealing, to enforce the idea that they shouldn't be doing damage?

Let's say the healing in a raid is covered 100% by the Dedicated healer. If it's possible to remove some healing GCDs from their load, by having the Support healer throw out some occasional heals like Indom or such (tools they would need to have, in order to get through solo-healer content like a 4man dungeon), then the Dedicated healer does what with the newly freed up GCDs? Presumably, damage. So instead of 4 healers who all do somewhere in the ballpark of 6k damage, it reads to me like this idea is 'lets have 2 healers that do 6k, and 2 healers that get saddled with more of the healing responsibility, and because of that only get up to 2k damage.' Now, this might sound satisfying at first, having a healer that is 'the dedicated HPS machine'. But this is meant to be for the benefit of 'players who don't want the stress of managing a DPS rotation', right? So, here's the kicker: with this idea, the point of failure for 'we didn't have enough healing' goes from two healers, to mainly being the burden heaped onto just one. 'Why didn't we survive this mechanic' changes from 'we didn't get healed enough from WHM, maybe also if we had more mit from SCH it'd help WHM keep up' to 'the WHM did everything wrong'. Especially when you factor in how in this tier, where a DPS missing Feint or Addle would often translate to 'where was shields and mits SCH/SGE?', and when you say 'I used them, you didn't use Addle' they say 'go again' and drop the conversation before the fault can properly be assigned. The healer is the first point of blame for a wipe, and this would just reinforce that perception like tenfold imo

Unless we're suggesting keeping mits on the Support. In which case, it's the Pure/Barrier split, renamed? I'm not getting it. But all in all it sounds like a design being written for a FFXIV that doesn't exist, and cannot exist, because people have formed a perception of how it plays. Over 8 years of 'healers need to pump as much damage as possible' is not going to be undone. Even if the gameplay is changed to try and enforce one healer to be 'the HPS bot' and one to be 'on standby to spotheal', people in this game will find ways to try to reduce the GCDs used by HPSBot, so they can do damage too. The only solution that comes to mind is to make HPS requirements tuned in just the right way, to have HPSBot's HPS be mandatory, and DPS requirements such that Support-Healer's DPS is mandatory. Enforcing the 1/1 the Pure/Barrier split is meant to. However, as with P/B, that would only work in week 1/2/3 prog. Once gear comes in, we're back where we are now. And it still doesn't address the fact that in more casual content like dungeons, if SupportHealer is able to heal through the EX roulette (and they'd have to be able to by design), HPSBot Healer's HPS is going to be redundant.

So while I can appreciate the sentiment behind the idea, I think it's just not feasible in execution.