My approach to multiboxing is how to get the most effect out of the least brain power--Ie, with healing I want a system that takes no thought when I'm under low to moderate pressure and takes as little input as possible in an emergency healing situation.

The broad strokes of my setup are four macros that go to my Paladins. If you arrange them top-to-bottom in ISBoxer and use 'World of Warcraft Macro Actions' instead of sending keystrokes and having the macros in-game, the characters will execute them in order, flawlessly. Here are the four macros:

/castsequence [@focus,exists] reset=3/combat/alt Light of the Martyr,nil
/cast [@targettarget,exists] Holy Shock

/cast [@focus,exists] Holy Shock
/cast [@targettarget,exists,noharm] Bestow Faith

/cast [@focus,exists] Bestow Faith
/cast [@targettarget,exists] Holy Light

/cast [@focus,exists] Flash of Light
/cast Light of Dawn

Over my health bars I have invisible clickbars such that if I left click on a character, it sets that character as the Paladin's focus. If I right click, it sets them as the focus AND activates that character's defensive cooldowns. As I DPS, the Paladins will prioritize Holy Shock, Bestow Faith, Holy Light, and then Light of Dawn. If they have a focus, instead that character will get ONE Light of the Martyr from each Paladin, then it prioritizes Holy Shock, Bestow Faith, and then Flash of Light on the focus target AND will skip casting Light of Dawn while they have a focus. I have a keybind I press to clear focus on all of the characters, and every time they change focus they reset their Light of the Martyr castsequence.

It's not perfect, obviously. If my target is stunned they drop target, and so the Paladins won't heal. Also, if the target isn't targeting one of my characters, I will do very little healing (in a PvP situation they'll sometimes be targeting other people). Also, when there are lots of PvP targets at once, the character who is taking the most damage may not be the one targeted by who I am DPS'ing, and therefore healing.

That all being said, it's pretty fucking elegant It takes zero thought for 90% of my play time, and when a toon is taking a lot of damage I just click their name and POOF, they're back to full health in one global.