What Content NEEDS Two Tanks?

[Gamemako]

Crystal Tower (No odd setups allowed, hee!), Extreme Mode Primals (Definitely Garuda anyway!). And usually King Moggle (Boss on MT, WAR/PLD on OT) and HM Ultima (Aggro trading for debuff stacks).

CT only really needs three of the six, but you can't queue with anything but 2 per group / 6 total. First room needs only 1, second room needs only 1, third room needs only 1, bone dragon needs only 1, Atomos needs none at all, Thanatos needs three, voidsent needs three, Behemoth needs two, Acheron needs one. GKMMXII needs one, Garuda EM needs one. Dunno about other EM primals.

[Lucavern]

Garuda EM needs one. Dunno about other EM primals.

Titan and Ifrit currently need 2, as they have a stacking debuff that increases the damage taken by the tank. It might be possible to outgear this eventually, but I don't see it happening at present. Titan's stacks fall off on occassion, I believe based on DPS-speed. Not sure how adds would work, but I think a dps might be able to manage them. Ifrit might also be really RNG-based with 1-tank, as from my experience the tethers during nail ended up targetting a healer and dps. This could become extremely painful if the tethered healer was also the Searing Wind carrier.

[Viridiana]

Titan and Ifrit currently need 2, as they have a stacking debuff that increases the damage taken by the tank. It might be possible to outgear this eventually, but I don't see it happening at present. Titan's stacks fall off on occassion, I believe based on DPS-speed. Not sure how adds would work, but I think a dps might be able to manage them. Ifrit might also be really RNG-based with 1-tank, as from my experience the tethers during nail ended up targetting a healer and dps. This could become extremely painful if the tethered healer was also the Searing Wind carrier.

This just got posted over in the Dungeons forum.

[Maelwys]

Thanatos needs three

For CT I was referring to the queue mechanics more than the fights, but it's probably worth pointing out that I've held aggro on Thanatos right through the entire fight (not by choice) several times... so it's definitely doable with one tank, just a bit trickier since competent DPS will peel him off you eventually. I hadn't seen that Garuda video before... a lot of the newer endgame content seems to have built-in Tankswapping mechanics (Ultima inflicts stacks at regular intervals, Titan inflicts them for the Table Flips IIRC, and I'm sure Ifrit is much the same) but it wouldn't surprise me if certain groups find a way around them via overgearing and/or buffing so much that they avoid receiving the stacks...

I suppose once you reach a certain level of skill and party makeup specialisation you could make pretty much the same point about not "needing" DPS classes for certain content... or even Healers