My raid group has 2 shadow tanks--Toth isn't an issue. (In fact, we leave him in Berserk for the whole fight)
That being said, the first few times we fought him, it seemed as though I was simply not able to survive it. (I was on the Zorn side, so I got Toth's first jump and Berserk). Attentiveness, as others have said, is vital here. And communication.
When Toth hits 90, 70, 50, 30, and 10 percent, he jumps. That's going to do a lot of damage to you if you're on Zorn when a flying Drouk lands beside you. So be ready! If your health isn't topped off, as a tank, it's time for defensive cooldowns. You have a bubble, resilience, battle readiness, and *possibly* a clickable relic. You know to stop DPS on Zorn when you get "Fearful" --that should help--and you're hopefully keeping kinetic ward up the whole fight.
Point is, it's *supposed to hit you for ludicrous damage. Because if it didn't then, the healers would be able to massively over-heal the whole raid, negating accidental aoe damage taken, etc. You could breeze through the fight and be sloppy.
The fight makes you, as a tank, search for a cooldown rotation to help your healers, and it's up to your healers to zerg-heal you on the jumps, and it's up to the dps to avoid damage from fearful, the scream, and the red circles in order to help the heals. Oh, and--switch bosses--that's happening, right? On the jump, each tank taunts the opposite boss. Because one you get the fearful debuff, you can't hit Zorn for 40 seconds without killing yourself.
Like a team fight!
Coordination is definitely key here--we call our 92%, 90% jump! Every time.
Welcome to swtor tanking!