A very easy way to do this, would be using a repeater region but to only use that for the targeting.
Give everyone the same alphabetical listing, so any given frame within the grid is the same target for each of your toons.

Then use mapped keys to cast whatever, at your current target.
The mapped key could easily round-robin with steps, something like:
1: Toon A, Toon B
2: Toon C, Toon D
3: Toon E, Toon A
4: Toon B, Toon C
5: Toon D, Toon E
That gets you two toons casting whatever you want, at the same target on a given keystroke.

Just decide how many toons you want to do a given action.
PW: Shield would be one toon casting per step.
The Renew might be two or five, or even keys for both depending on the situation.



The Pallies with Exorcism/Holy Shock, aren't lacking for DPS.
They are lacking AoE heals, but could easily be run with Focus Healing, as per Mosg2's DK Manifesto, or a system of round-robin heals.

The Shammies wouldn't have nearly as much offense.
But would AoE heal a lot easier, more toons in that regard.
And they have greater fear protection, and anti-swarming (short range AoE) tools.