Use a healing mod such as VuhDo on the healer and lead with the healer. Much easier to heal your group with the mouse and dps with the keyboard. /assist party1 or your tanks name works fine. If you use FTL then /assist with the healer and have your shamans assist the healer. It is an extra step but you have the chance to stop casting if the tank targets something in the next pack that he shouldn't. Also having the current target up on your healer lets you do some dps with the healer on easy healing segments.
Edit: I do this when I run with my tanking buddy. It is a blast not having to worry about tanking for a few runs. Just blow stuff up and heal.
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