The Draenai racial to spell/melee hit doesn't stack.

My guild runs multiple ZA's and Karas a week. If it is acceptable to others I plan to bring two three chars at a time to the different raids. I can't really think of too many high end raid encounters where dual-boxing would be a problem. Kael would definitely be the toughest, having to loot multiple weapons(and sometimes they are a pain to target just with one char, especially the mace) and problems with gaze.

More than the mechanics I think the biggest hurdle to raiding as a multi-box toon would be loot drama and such from other people.