I got my priest duo up to level 42 and a half before 2.3 hit, fooled around with world pvp and gimmick raids and gimmick RP but hit 70 in 12 days 3 hours /played.

After 2.3 hit there has never been a better time to start multiboxing, I started an moonkin duo the other day. If you start boxing, try to aim for three characters over two so that you get a party bonus, else the duo will simply split all exp by 50% and it might lag you behind as you go about your quests.

Also, remember this:
Once 58, do the first batch of quests in Hellfire Peninsula for gear, then return and finish off Azeroth, as much of it as you can, even the grey quests! People have reported reaching level 64-65 this way before finishing Hellfire Peninsula, and having unbelievable amounts of gold as a result