This thread over on the forums starts off a little silly (it's a complaint about Kael'thas being both in Tempest Keep and Magisters' Terrace at the same time), but later on gets into some really interesting commentary and thinking about just how the lore and story in this game works. Obviously, there are no problems with having Kael in two places at once (as Bornakk says with an epic burn, a character can be alive on page 1 and dead on page 10 at the same time), but it's interesting that that's not always what happens in the game. Griftah, for example, was never in two places at once, and neither was Hemet Nesingwary -- both of those characters had progressive lore storylines. But Kael, like Thrall, has a storyline that evolves as a character moves forward in their own storyline, and so Blizzard has placed him in two different places at once, and asked players to move their own characters to see the storyline in order.
Interesting stuff. Scuzz makes a particularly interesting point in the thread -- the World of Warcraft really does revolve around player characters, not NPCs. If you discover through quests that Onyxia is actually pretending to be a woman in Stormwind, all of the NPCs your character meets will be shocked. It's a sign of Blizzard's talent at game creation that something can be both known by most players and discovered by one player almost every single day.
Of course there's no problem with Kael being in two places -- I would rather have my character move through a changing world than have Blizzard worry about making sure "reality" was kept correctly. But it's an interesting line being walked very well -- if you do the two instances in order, you get a much better story than if we were simply fighting a generic third boss in Magisters' Terrace.