Wouldn't work, let's go through each content.

Dungeons, 1 tank, always MT.

Normal trials/8 man raids, you can enter either on MT spec or OT spec and have no guarantee on being on the right one, 50% chance your damage is neutered for no reason.

Alliance raids, dependant on whether the raid requires each tank to MT at some point, you will have the wrong spec.

Extreme trials, Savage and Ultimate. With tank swaps, you will be MTing/OTing at some point, calling it even, 50% of the time you will be less effective, just because.

This is before we get into the logistics of having to keep swapping depending on the content you do, where, for most of it, it is a coin toss whether you have the right one or not.

So, for me, it falls flat before you even start looking at how you would change the jobs.