Before EW there was WHM which was a HoT healer, SCH which was a shield healer, and AST which could choose between either. I played WHM and AST, because that allowed me to always fully utilize my kit. If I was on AST and got paired with an SCH, I'd choose diurnal to avoid shield overlap, otherwise nocturnal.

How we have SGE which is a shield healer, and AST is permanently diurnal. Any combination of SGE and SCH will be handicapped because their shields don't stack. SGE is even worse off than SCH due to addersting. I don't know how exactly it works with another shield healer, but chances are the stack is not granted if the shield from Eukrasian Diagnosis is ovewritten by another shield, because that would be exploitable to gain stacks quickly and even out of combat.

Some time ago SE removed the damage type specific debuffs from dps jobs to make more combinations viable. Could something be done about healers so any combination works?

PS. I realize that duty finder content where I'm randomly paired with another healer is easy enough that suboptimal performance won't matter. But I also have a friend who plays SCH and I'd love to be able to use SGE when playing with her without either of us feeling like we're stepping on the other's toes.