Most of your healing will be on the Paladin.
But you'll want some group heals as well.

I'd have a few heals set up where you assist the mob.
Renew, Flash Heal and Greater Heal.
For damage to the pally who is tanking, this will work fine.

To do this, use the assist method of choice.

#show Renew
/assist Pally
/cast [help] Renew; [help, target=targettarget] Renew; [target=player] Renew

Can do the same for Flash Heal and Greater Heal.
The first condition will heal the pallies target if that is friendly.
If that fails then it attempts to heal the target of the pallies target (which should be the pally).
If both conditions fail, it defaults to healing the priest.
You could change the last condition to [target=Pally'sName] Renew, if you'd like.

Prayer of Mending is extremely efficient, you'll likely want the priest to cast this on the same keybind as your DPS toons and tank keys.
If your DPS macro and tank macro are on Key 1, put something like this on key 1 too:

#show
/target pally
/castsequence reset=combat Prayer of Mending,,,Renew,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

Give it as many comma's (do not casts) as you think you'll need.
Basically you don't want this macro to fire off, as your DPS toons are DPSing, if someone else needs a heal.

A lot of people will have a heal to specific toons as well.
Numpad 1, might be heal your Lock.
Numpad 2, might be heal your Boomkin.
Numpad 3, might be heal your Hunter.
Numpad 4, might be heal your Priest.

You'll likely have Prayer of Healing on a keybind as well.

Basically with a set up like this, you can:
a) Renew, Flash Heal, Greater Heal on Pally's target or target of pally's target.
b) Get Renew and Prayer of Mending on the Pally as part of your DPS macro.
c) Have the ability to Flash Heal a specific toon quickly.

Everyone has their own system, something will work for you.