I single-boxed my first warlock as far as 70 and I found that I didn't have much downtime anyway. Dot, fear, drain, life tap, repeat. I don't think a priest tagging along would've made my leveling that much more efficient, though having the occasional resurrection (in the unlikely event that something which killed my lock didn't deal with the priest as well) might've been convenient.

I five-box but if I ever had to drop down to two accounts, I'd go with mage + demo lock for focus magic + arcane int + felguard +spell-on-crit. Maybe make both of them engineers for synchronized bombs, rocket boots, etc.