You can do a lot of assist healing.

/assist Tank's Name
/cast [help] Renew; [help, target=targettarget] Renew

This will Renew the friendly target of the tank, but if that target is hostile, then it will cast at the target of the tank's target, which is likely the tank but possibly another dps who overnuked.

I like to have one key for having the other shammies assist with heals.
They do the assist heal button, for lesser healing wave.

The main healer (shammy) has Healing Wave, Earth Shield, Chai Heal and Riptide as assist heals, along with toon specific heals too.

On my other group, the (priest) has Renew, PW: Shield, Penance/Flash Heal as assist heals, Holy Nova on its own key and Penance/Flash Heal for each individual toon on a G-key. My Penance/Flash macro is something like: /castsequence reset=7 Penance, Flash Heal, Flash Heal, Flash Heal.