Unless the new raids feature a lot of back to back tank busters within a few seconds that wouldn't be a problem, even now you don't really have to topup tanks after a tank buster since they'll only be taking auto attacks for like a minute until they have to face another tank buster. Usually they could be topped up by a regen and pet heals. You can coordinate tank mitigations and self heals to help, as well as tank swaps. Forcing healers into two groups would be going in the direction of the current tank meta, where there are "main tanks" and an "off tank" due to dps skills that proc on getting hit (parry/block/taking dmg), but that doesn't really work since the devs must make the off tank type tanky enough to tank every content in the game, making the off tank type good enough for mitigating damage while bringing more dps/utilities.We still see drk/war as the optimal composition because of the LB penalty, if we have another off tank type like warrior, the optimal meta may include both of them. The same will have to hold if you have "full healers" and "half healers", the latter will have to at least be able to heal every content in the game. They probably can make that kind of system work, where there are two types of tanks and two types of healers, but considering the info they released about stormblood they seem to try to move away from that, since they mentioned stuffs about warrior being guaranteed a spot in raid groups for now and they want to change it.
You also have to take into consideration player skill gaps, maybe an average group needs a full healer and a half healer to fulfill the healing requirements, while a really good group can do the same with two half healers. As of now the gap between skilled and average groups is quite insane, the amount of heals required to keep a group of skilled players alive is so much less than the amount done by healers in average groups, and that's where they get so much time to dps. Pushing for more healing requirements would make the top healers do less dps and heal more, as well as make the average healers unable to keep up with the healing checks.