These are things I noticed during the fight on 16 man take them with a grain of salt. I'm not positive on the mechanics, but these are my observations is after ~4 hours of attempts. I'll update as I find more information, there were a few minor bugs that involved Zorn double jumping.
Zorn and Toth
Note: The percentages are what I roughly noticed while fighting the boss.
Our initial positioning was two groups of dps and healers one on Zorn and the other on Toth. The bosses are spread apart each on either side of the cliff. melee DPS stays on Zorn 100% due to Toth's melee range AOE damage. We had heavy DPS on Toth so we could stay slightly ahead of the Zorn so we know when to expect abilities. The abilities seemed to occur during certain percentages of boss HP and activated when either boss hit the health percentage. (The 90% mark on either boss is solid.)
Ability Set 1 Jump and Scream Debuff
The first Jump/Debuff ability occurs at 90%. The Fearful debuff increase damaged taken from Toth, and I believe the jump just brings the mobs together to cause a enrage so they have to be split up. I saw 3 of this ability during our 5% wipes.
We discovered that the Fearful scream can be out ranged. We had our ranged DPS run out to avoid getting the debuff. Apparently if you continue to dps Toth with the debuff, he will eventually do his slam that instant kills anyone < 40 meters away from. If you receive the fearful debuff it was best to switch to Zorn until it fell off.
This is where timing seems overtuned or we had bad positioning. The tanks ideally swap mobs after the Jump/Debuff ability. Once the abilities occurs again, the debuff duration does not end on the first tank so when the tanks swaps, the tank takes extremely large amounts of damage. Initially we thought to slow DPS so we could transition them on our own terms, but we ran into the enrage timer on multiple attempts. Perhaps Tanks CDs must be used during this phase since the debuff falls off only a few seconds later after the swap.
Another Issue was healers getting the Fearful debuff due to having to be in range to heal the tank so a coordinated healer swap with tank CDs are necessary if healers get debuffed.
Ability Set 2 Zorn Enrages and Thoth Throws rocks
This happens at roughly when either mob hits ~80%. Toth stands still and will start to throw rocks and Zorn "enrages" (does more damage not instant kill). Anyone < 30 m away will get targeted for rocks, If you have the Fearful debuff and get hit by a rock, it is pretty much a instant death. As long as you are 30m away you can continue to DPS Toth. I saw at least 2 phases of this during one fight.
The tank, who is tanking toth, can also run away during this phase and take no damage; therefore, a tank swap does not have to occur. The catch is at the very end of this phase Toth will apply the Fearful debuff again so dps will have to run out to avoid it. We could not consistently avoid this debuff with random people getting it and not getting it. I think its because the tank decided to hang out with ranged to avoid damage so if Toth takes one step towards us he is now in range to apply the debuff.
For Zorn just keep the tank up, I think the tank tried kiting Zorn to reduce some damage, but it wasn't necessary with minor CD and good heal.
Ability Set 3 Rock Spikes (Zorn) and Sonic Scream (Toth)
I want to say this occurs at ~50%, I saw about two of these phases during one fight.
The red target icons appear on ground, retard check.
Sonic Scream is "unavoidable" AOE damage that ticks for 2k ~ 3k (I specd into damage reduction so it may be for more). I put unavoidable in quotes because some people said they could out range it, but I ended up not being able to avoid it at 40+ meters away. Saved medpacks for this phase and healed myself while trying to pump as much dps as I could. Sorc DPS bubbling other members also helped the healers and less and less people seemed to die.
It then cycles back to ability set 1, not entirely sure it random or not, but it seemed like it cycled the abilities in a certain order 1, 2, 3 repeat.