What Content NEEDS Two Tanks?

[Contemporary]

The only content that requires two tanks would be the new 2.1 Extreme Modes + Ultima HM.

Sadly, you can out gear every single Turn in Coil (Even Turn 5, there are videos all over Youtube).

[Maelwys]

What Content NEEDS Two Tanks?

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).
The Entirity of Coil has been done with One Tank - it's pretty easy for Turn 2 if you're using the 0Click method on the final boss. For Turns 1 and 4 it's a lot trickier.

Honestly Turn 2 and HM Ultima's "Normal" methods (Aggro trading) are probably the only situations where I've felt there was no designated MT/OT and both tanks were reasonably equal.
That said, I enjoy OTing certain content like Turn 4. Proper add-handling can take more skill than just keeping your Face in front of a Boss's Fists.

[Lucavern]

The need for 2 Tanks would be Turn 1,2,4 the new Extreme Modes Garuda, Titan, Ifrit. King Mog could be maybe outgeared, for lower Itemlevel groups most likely 2 Tanks.

Turn 1 doesn't require 2 tanks. Overgeared tank can let them remerge right away and kite for duration of fight. DPS has to watch threat and tank needs to be good at mobile threat-gen without getting in melee range, but its doable. Our off-tank was restarting router right before Cad, and we decided to try anyways. Ended up beating it right about same time he came back, laughing while he was outside the room.

Turn 1 ADS may require 2 tanks, but I think an over-geared dps can tank the adds. Never tried, but I think its possible. Also, turn 2 only needs 1 tank if you use the enrage method.

[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.

[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

[SpookyGhost]

Turn 1 ADS may require 2 tanks, but I think an over-geared dps can tank the adds. Never tried, but I think its possible. Also, turn 2 only needs 1 tank if you use the enrage method.

Honestly, the overgeared DPS would be better off tanking ADS lol

But yeah, all of Coil CAN be done with 1 tank. Turn 2 being the only exception if you don't want to cheese it with the enrage... though I think it's still possible with SSing the stacks. Never tried it, though.

Garuda EX can be done with 1 tank, but I don't think Titan EX or Ifrit EX can be done without 2 tanks. Same for Ultima HM I imagine (but I haven't touched Ultima yet).

[Zoomie]

Turn 2 being the only exception if you don't want to cheese it with the enrage... though I think it's still possible with SSing the stacks. Never tried it, though.

Cheese it with the enrage? I'm not familiar.

[whiskeybravo]

I don't think you can single tank Ultima HM. Even if DPS could technically survive through a couple of stacks, they have no Provoke to peel hate of MT. Any amount of gear going to let you eat over 15,000 damage? I don't see how it would work unless MT just focuses on holding on to hate by a thread.

[Gamemako]

Cheese it with the enrage? I'm not familiar.

Tick timer down to zero, let it enrage. Basically starts hammering everyone for 3500 damage a shot, but you can just spamheal through that much damage and kill the ADS without dealing with any mechanics at all. When everybody is overgeared, it's an easy way to avoid all coordination needs and just zerg the fight.