Tank A (in tank stance) has 100% (top) threat.
Tank B (in dps stance) has whatever threat, doesn't matter.

Tank B Provokes, Tank A shirks. Tank B now has 125% threat instantly.
Tank A Provokes back, Tank B shirks. Tank A now has 156% threat in 2 gcds, if required.

Basically speaking, the tanks can, whenever they want to, up to once per two minutes, give themselves a free 56% threat increase on current threat. It doesn't rely on the generation of threat from the offtank at all; the only number that matters is the current threat of the main tank. Provoke sets the offtank to the same value, then shirk increases from there.

You don't have to use them together either; even just using on tank swaps effectively cements the new tank without any issue whatsoever. As soon as you see the enmity change from provoke, you shirk, and they are 100% tanking.