While we wait for someone who knows what they're talking about to answer your question, I'll throw my two coppers in.

I'm pretty much in the same boat. Two accounts. I'll likely run two machines tho. I haven't dual-boxed in WoW at all, but I've played 4 accts in DAoC. What I found most fun was sets that had good synergy. I'm looking at starting with Warrior/Priest but I've given thought to Druid/Warlock for the added flexibility. I don't know how "uber" or efficient that would be, but I'm more concerned with what is fun and interesting to play. As for the Warrior/Priest combo - one guide says that is pretty slow, but I'd think its fairly reliable and a good place to start anyway.