I haven't duo'd 70-80, but I'd guess it is possible, given enough intestinal fortitude. I'd guess you could maintain 200-300K/hour questing (more if you know the routes, tricks, etc.), which means roughly 5-7 hours per level from 70-80. So, you could pull that off in a week if you played 8-10 hours per day. 60-70 should go much quicker, especially with all the pre-purchased turnin quests available. Agiain, it would take a real gut check, but I think it's possible.