Well, for leveling I'ld suggest speccing the resto shammy to elemental and heal with the pally (click2cast is a great mod to temporarily heal with just your main char)
even lowlvl instances aren't a problem like this,
all aggro except the one under fire will go to the pally, but since he's getting hit most, keep a way to heal with the shammies
I have no clue how this will work out in higher level stuff tho
then again, they mow down mobs so fast you won't feel a need for the extra dps

I'm sure the attack till death has been discussed before,
I'm not sure, but maybe something like:

/cast [harm,nodead] spell
/stopmacro [noexists] [dead]
/assist party1
(bleh, I've been out of this for to long, I'm at work atm and can't be certain about if it works like this or not)