In my 10-man we have the following: Healadin, Resto Shaman, and Resto Druid. Here's how we do healing on Loatheb

The resto shaman, tree, and all the casters stand together. The healadin (me) stands outside of the range of Glyph of Holy Light. I set my focus as the shaman and Beacon of Light myself. The Shaman chain-heals the melee, I Holy Light the Shaman and the casters (thus healing myself) and the tree heals the tank or whoever needs it, with our Elemental Shammy throwing down extra chain heals as needed.

Occasionally, depending on who takes damage, you can change your focus to someone else in the caster group, but I wouldn't take focus off the other healers if I were you. If you time your casts right, you can get off two Holy Lights and a Holy Shock in the time the healing window is open.

for loatheb, we usually have a holy pally and a holy priest (me) and the holy pally cannot do much in the short time in between necrosis auras, but by timing my prayer of healing (I ask to be put in the caster's group) then my CoH on the MT and a Shield, I get the entire raid topped off. The Pally just tries to keep up, but he's usually throwing flashes on my 'leftovers'

but him and I are on par for the overall healing done in a single run, so Holy P's are a good team, but for loatheb, a single holy priest can do it...