I leveled 3 toons to 85 solo, and am working on the other 7. Leveling them at the same time may feel slow, but it is much quicker overall than leveling them solo.

I have to disagree with Merc on the zones - call it personal preference. Vash is not great, but doable (less ganking there is a plus for me). I like Hyjal as a zone more than Vash, but the ganking made it a painful place to try and quest. Uldum is awesome especially if you like Raiders of the Lost Ark. Twighlight quests are painful. Soooo many collections. I only did the complete zone once on the three toons and that was enough. Deepholm was a lot of fun.

What I did on two of the toons is go to the starter zones until i was high enough to get to Deepholm, do all of Deepholm, then go to Uldum or Twiliight.