As many have suggested, having 2 tanking specs really isn't an optimum way to go, especially if you have 2-3 (or even 3-4) dedicated tanks of varying classes in your guild. The reason I say this is because at any given time, especially for warriors, your class will not be strong at tanking any given situation not because of your spec, but because of your class mechanics.
We've had a lot of different encounters in the game so far, each requiring a varying number of tanks, and a few encounters favoring a very specific type of tank. The easiest example to bring up are encounters with one tank fights. Which tank is going to go in there? If you have two tanks, a warrior and a druid, and the fight is high magic burst, it doesn't matter which spec the warrior is, the druid is preferred, thus making the warrior a burden to the raid since he very well can't DPS effectively in either of his two tanking specs. I'd like to believe that Blizzard will eventually balance the tanking classes, but it certainly won't be by the time 3.1 hits!
As tank captain of my guild, I'm going to have a lengthy discussion with my tanks about the type of tank service they want to provide to the guild, and then have their tanking spec customized to serving that role for the encounters we're going through. When they're not needed in tanking responsibilities, then they must DPS effectively. Even now, I'm very hard on my tanks when they have a sub-par DPS set. One guildie joked over vent once that one of our tanks was doing such low DPS that he was healing the boss (I lol'd) but come progression time, we can't afford that kind of sub-par performance, and it'd be a hard argument for him having 2 prot specs and wanting a tank when it's clearly an encounter suiting a druid tank.
tl;dr, I think if you have two tanking specs, then you are nearly worthless in encounters where you are not tanking. Being replaced for tanking for specific encounters is a given with our current unbalanced tank classes, so it's always better to be prepared to step down.
Stay strong. Stay smart. All heart.