Make a macro for swiftcast raising too, just something that announces in Party Chat that you're raising X to avoid both healers blowing raise on the same person.

Cure 3 > all for healing a bunch of people who are tightly grouped up and in need of healing.

Also Esuna, Esuna, Esuna, Esuna. Cleansing is one very important part of playing support that way too many Healers neglect to do.

A lot of it is just general practice really, once you've done a dungeon enough times you know what the heavy hitting stuff is and can pre-plan accordingly around it.

Also once you get to 8 man content, you do get to hold the balance in your hands. If someone is constantly getting hit by every bit of avoidable damage in the fight, unless they're the tank it eventually becomes counter-productive and far too mana intensive to keep them up. In the rank and file of priority targets, they will be rock bottom, and it's okay to let them die for it.