On my shamans, I just set the tank as focus, and cast LHW on the focus target with a button. I have a LHW button for my main, since him falling over would be the most detrimental, and I have a chain heal button where each toon just casts it on themselves.

In heroics, usually you wont really get a certain toon getting focus fired for very long, but if it happens, click his frame and cast a heal on him manually. Usually if anyone except my main is getting hit, everyones getting hit a little, so i just chain heal on each toon, its smart enough to then leap to the lowest 2 targets, so the characters that need the heals the most will get the most healing from it.