I would separate the Shaman and Druid, because they're both capable of ranged (caster) dps and heals as needed.
Play the warrior and rogue as the active toon, break follow on the ranged toon a short distance from where you engage.

Personally I would go Shaman + Rogue and Druid + Warrior, because the rogue will be a lot more fragile then the warrior, and earth shield can off-set this to a large degree.

Windfury + Strength of Earth totems, and support "pew pew pew" and your team will have massive dps for two toons.
Similarly, warrior (dps spec) with support dots/wrath will be good dps, I'd consider Dreamstate on the druid.