Some people have this strange idea that it's actually good to have two "main tanks" and two "off tanks", but, this would be DEFCON 1 for tanking for several reasons.
First, one tank would have the lion's share of aggro generation moves, as they'd be the "Dedicated main tank class", while this would impede several things. First, swaps would be completely ruined by this, as the main tank class would be far superior at generating aggro, which would lead them to constantly ride the "dedicated OT" for threat. If the "Main Tank" dies, then how does the OT, with less aggro resources (as they are the OFF tank, remember?) pick up from where they left off? All of that threat disappears and you get into hate issues.
Secondly, the dedicated "Main Tank" would have MORE mitigation options, as they are the MAIN tank, right? What does that leave the off tank? If fights are balanced around the "Main tank" holding the boss more, then obviously there are going to be more tank busters. If the main tank dies, how does the Off tank pick up the slack with less defensives, and more offensives, naturally (I mean, how else would it be?).
Thirdly, we would just see a REPEAT of the Tanks using damage accessories because they can thing, which would lead to comps of two "off tanks" tanking instances to pump out as much damage as possible, breaking the entire point of the "2 MT 2 OT" thing. I honestly don't know if that's the way they play on JP, where there's a "dedicated MT", Yoshi P was (perhaps it's just rumors) reported to have said that the WAR needs more competition as OT, which is why we are getting GUN.... like what? On NA, WAR is almost always the MT by default because it pulls, and pugs are too lazy to do swaps most of the time unless it's forced. This would only lead to the "MT" tanks being shunned for the higher dps "OT's". It happened with i270 accessories.
Tanks are going to optimize and gravitate to the most damage possible, so these three things when taken into consideration, are only going to get far, far worse if the 2 MT 2 OT thing is implemented.
I honestly hope it was just a joke.
Both tanks should have AT MINIMUM:
Excellent ways to pull the boss (achieve this in different ways if you want to be unique, NO TANK should NOT be AS effective at pulling as the others... Full. STOP.)/Snap aggro
A FULL suite of mitigation, how this is achieved is up to the devs to make it creative, but no tank should be the "welp, guess I have nothing" tank as for as mitigation goes.
The ability to achieve similar DPS without a clear winner (WAR).
Unique utility for all tanks, how this is achieved, can be unique and rewarding to each tank complimenting a party in different ways. Currently, DRK is the tank that you play because you WANT to, not because it's good. PLD offers tons of utility, WAR has shake and great DPS, DRK is just...there.