Had a support ticket I thought was funny.
Thought the underlined portion was funny, like an oldform letter: "Hey there Bullvena!
Game Master XXXX here to help :D!
There are definitely situations where your characters will join character specific phasing areas, in which only one of your characters will be playbale at a time, rather than in a group effort.
However, If you have completed the same quests on each character, typically phasing won't be a problem as most phased areas are not character specific. Hyjal for instance has a lot of character specific phasing as you are helping those immortal aspects of nature.
Places like Vashj'ir however, once you go through the character specific quests at the beginning of th zone, will allow you to phase as a group.
I cant say which specific areas later on in the game will cause you this problem, as we don't have a ton of information in regards to which areas are specific to character phasing. However, there are a lot of multiboxing leveling guides online, and even some addons.
I did a quick search for some addons that might help and I found one that might catch your fancy. http://www.curse.com/addons/wow/hydra
As a side note though, you could also level up in battlegrounds. Places like Isle of Conquest and Alterac valley grant a pretty good chunk of EXP when you win, and with a multiboxer on the team, typically you will win hehe. Gotta love that epic nuking ability :]
I hope it helps out and be sure to let us know if you run into trouble in the future! Take care now!"
