The times I've done swaps for my runs have been as follows...

PHASE 1
No switch -- first tank holds throughout until first geocrush, regardless of how many stacks, even if it's just 1.

PHASE 2
-Switch immediately as he lands to geocrush.
-Switch immediately after the landslide after he busters for 2nd stack.
-Switch immediately after he busters for 2nd stack during gaol animation.

Unless dps is slow, this should be the last necessary swap of phase 2. MT at the time should only go into heart phase with 1 stack, maybe 2. Unless, somehow, you get to 3 stacks, a switch isn't needed.

HEART PHASE
No switching. Stacks reset very early on into phase.

FINAL PHASE
-Switch any time that's practical and safe before the 3rd buster at the start of the gaols. If the adds are still up at all when Titan turns to start gaol animation, pop them cooldowns and be ready to eat a 3rd stack.
-Depending on when the first switch of the phase happened... if the first switch was at 2, switch immediately after double plumes -- only about 5-6 seconds to effectively make the switch. If the first switch was at 3, switch immediately after landslide/row bombs.
-Hold aggro through end of rotation and into start of next rotation, as tank on him will only have 1 or 2 stacks before they reset.


Doing this method for the final phase has helped my usual co-tank and I have our roles on pure lockdown and cuts out all guesswork during the last phase by having us literally repeat exactly what we just did in the first rotation for the second rotation.