Druid+anything really.

Basicly, as a druid you can be a healer when needed, you can be ranged dps when needed and you can tank when needed and all pretty damn good too. Yes you would respec a lot, but tbh, if I was dual-boxing I would go druid+something else (Probably paladin, shaman or priest).

2x hybrids = win, imo.

When you level two pure dps classes, you will still be limited to do pure dps only in endgame. And tbh, the fun with multiboxing is being able to take care of more then just the dps, or just the healing, or just the tanking.

Im a quadboxing ele shaman, tanking is out of the option in wotlk, only few bosses are earth elemental farm nukable, but most do require a tanking class. But I can still cover dps and healing 100%, and ofcourse able to keep standing in PvP. Whilest overall it is not -that- hard to control. Still, pve wise it doesnt matter if you go multiclass or 4x shaman in my experience. The hard part for pve is moving about and spreading out during encounters while being as effective as possible. And that's a feat to overcome regardless of the box set up you have. (having more then 1 melee adds yet another dimension and toughness to it, I would not suggest to go more then one melee ever on your first box, and maybe try it out as an alt box some other time.)

As far as the diff between playing for example 4 or 2 shamans. I personally find it boring to play only 2. You give a lot of responsibility to others in parties, whilest with 4 you, and you alone, are a great deal responsible for what happens. And the harder the fights get, the more I enjoy myself trying to beat them. With two it's almost the same as playing one. There is little extra dimension by tracking two characters, movement wise. It does add more then playing one, but hardly. So yeah, for dualboxing, two different classes would be most fun.