Kel'thuzad had that, though I wouldn't call the Guardians of Icecrown useless, as they had as much HP as Kel and were technically impossible to kill without highly risking a wipe. It's really all about context.
I'm guessing that's on me for assuming people easily understood the concept of having multiple targets to control during a fight.I read the first paragraph of your response and gave up trying to understand what you were on about truthfully.
*takes long breath*The concept of the four horsemen is that you have four bosses. Two of them stationary and two of them mobile. The stationary pair would stay in the back of the boss room and needed to have someone within range of them, otherwise they would spam a nuke that would hit everyone in the room for huge damage. The mobile pair needed to be held by one tank each.
Lady Blaumeux & Sir Zeliek = Stationary
Thane Kor'thazz & Baron Rivendare = Mobile
Anyone within 45(!) yards of a horseman would get a stacking debuff called a mark (Rivendare yould give you Rivendare's Mark, Zeliek would give you Zeliek's Mark and so on). The marks did more damage the higher the stack it was. The amount of damage could be releably healed until around 4 marks, as higher stacks would hit you for more damage and would eventually one-shot you.
The point of having two tanks is to have Thane and Baron far enough that the tanks and DPS on each would only be afflicted by one mark. At around 4 stacks, the tanks and the DPS had to switch targets and drag them apart again to allow one stack to drop and start getting marks from the new target. So if you're tanking Baron and I'm tanking Thane, at 4 marks we would switch so that you get Thane and I get Baron, allowing your marks from Baron to drop and getting marks from Thane (and vice versa for me).
Try reading the other examples I mentioned, please.
PS: I don't put much stock on 1.0 encounters aside from Garuda and Nael, as I understand it's mostly hold-over content and doesn't have much in the way of mechanics outside of adds and super-one-shot moves. We haven't seen a proper encounter that really requires a second tank, but we may get there in 2.0. I hope, anyways.