There are 3 ways I can see:
1) Tanks try to stay about equal in aggro - similar gear, +threat abilities. Use Provoke, and other lesser (they are ALL lesser abilities) to move ahead of the other tank, slow/stop DPS if one looks to be too far ahead.
2) Provoke AND Confuse type abilities to actively swap. Relying on correct order of abilities.
3) One main tank, the other one just temporarily gets aggro when the main one is about to hit 0 stacks and cark it.
The main issues are cooldowns, and hotbar space.
Provoke has a 40sec cooldown, but increases your threat to the top of the tree, so as long as you keep doing threat stuffs, it doesn't matter how far behind you are.
Mass Provoke forces the mob to attack your for 5sec, 50sec cooldown, no increase to your threat. Art of War - same deal (5sec/45sec). Raging Volcano - same again (6sec/45sec).
Unlike Provoke, the detaunts do not drop you a position, but by a percentage of hate. Mass Confusion (33% / 30sec reuse), Confusion (50% / 30sec reuse). So the main issue with them, is they can drop you past both the other tank, AND some overachieving DPS/Healer.
For each of the styles the Timelines are:
1) Competing Aggro:
Disadvantages - When Provoke isn't up, the swap is not guaranteed to last longer than 5-6sec, so they have to be close in gear/style. Healing will get harder if both have high stacks.
Tanks cannot go all out on dps (contributing their smidgeon to the raid).
2) Ability Swaps:
Even with dissimilar gear/abilities Swapping is clean
Both tanks can keep DPSing
Tanks each do 2 position swaps in a row, so if one is slow back from Drakes, its not an automatic wipe.
Disadvantages:
Needs better coordination than 1) as Tanks are going down in threat too
Ubernaughts get around this by making the 1st phase last <75sec ....
3) ePeen tanking:
Secondary tank just uses whatever temporary taunt he has up, whenever Primary is about to die. Healing is hard.