See, the evil person in me would strip healers of attack spells, sans maybe one. You know, solo play and questing are still a thing. I'm not unmerciful. One that would be horribly MP draining to use in a fight (think Holy and cost stays a static amount based on MP- similar to black mage spells) and accuracy cannot be melded into healer gear. To make sure people didn't circumvent the accuracy ban, make healer a new Disciple- Disciple of Mending. Then make fights harder to heal that actually require both healers and possibly give them debuffs to apply as a means to give them something to do if there's no major healing needed at that current point.

I'm only slightly evil. I swear it.