I would not put the healers hotkeys on the same keybind as your dps/tank spam.

Let's say you don't need any heals at the moment.
Well, you're wasting mana healing full health targets.

Let's say the healer needs a heal, but you're clicking a dps/tank function.
Well, unfortunately you are already casting a spell so are not free to cast at another target.

Personally you I'd say you have a few good options.
a) Have hotkeys for the healer to assist off the tank.
/cast [help] Heal; [help, target=targettarget] Heal.
b) Have specific keybinds for specific characters.

I do both options actually.

If my tank targets a friendly, then my healer can Riptide, Healing Wave, Lesser Healing Wave or Chain Heal that target, even if they're not in my group.
If the tank is targeting a hostile, then the first condition fails, and the heal goes to the target of my tanks target, which is whatever its whacking at - ie the toon that needs the heals.

Then I use G-keys, which are just set for obscure keybind combinations.
And have wow macro's on those keybinds to Healing Wave or Riptide the specific toon.
I've even gone into Jamba, and configured the (I believe Jamba Me), so when a toon goes below 66% life, Jamba raid warns the active (master) toon, Vysha hit, press G7.