As I have not personally experienced 8 man content yet, my specific question is what is the point of having two tanks on content where there is only one boss, such as the Primals? Are there mechanics where two tanks need to absorb damage (like Patchwerk in WoW) or is incoming damage so great that you need to Provoke off one another?

I know Garuda had copies that spawned during 1.0 so you needed a second tank, but Ifrit and Titan seem to primarily be a single mob fight, from what I've seen of the normal modes anyway.

So basically, why would you want two tanks on these encounters with only one target?