As with Miles, I wouldn't change a thing TBH. All 3 healers face issues when doubled up, I have it in my head that ASTs can't stack regens for example? Could be wrong on this one tho!

In challenging content WHM / WHM is actually more problematic due to the complete lack of party mitigation. I've cleared O4S as double WHM, each and every Almagest was 'interesting', particularly the one right after GC Omega where you can't just sit there and drop Cure III bombs through it.

In trivial content where you're likely to end up with doubled up healers, it frankly doesn't matter. Any of the 3 healers could solo heal it with baseline gear and a modicum of attention.