I think it's a decent idea to experiment with.

I would probably go with a hybrid DPS/Healer spec on one of the healers.
Or potentially on both.
So you have the option to heal with either or both, and can still do some DPS with them too.

I'd personally rather have multiple healing classes then the same.
If you want two healers, consider Resto Shaman (Earth Shield) + Resto Druid or Disc Priest.

Or just stick with the model: 1x Tank, 1x Heals, 3x DPS.
But pick DPS toons who have healing capability or buff the healer in some way (Innervate, Mana Spring, Replenishment, etc).