This might be patronising, but are you using taunts to take aggro on saurfang, or are you just trying to out threat him with a straight rotaion?
You should be using Hand of reckoning, or Righteous Defence for encounters that need taunt swaps, not trying to out aggro him, becuase for most Tanking Classes have some sort of Reactive damage that's only threating when they are being attacked (Rune Strike, Revenge, Holy Shield, etc) so unless you severly out-gear/play your co tank, or they throttle back on their TPS should not be able to take aggro on something they are actively building threat on.
Taunts (such as HoR, RD) are key to fights like DBW, they do 3 things; firstst they make you the primary aggro target, secondly they match your aggro to the person whose highest on threat, and thirdly they force the target to attack you for a certain amount of time.
So when the encounter requires you to tanke aggro off another tank, use a taunt, don't compete against each other for threat.
Presently, i find i run with some really bad tanks who tend to taunt on CD, and have no clue if they lost aggro to another tank or a dps, and tend to taunt on CD. This has biased me against taunting for the large part; i consider taunting off a non-tanksomeone a persoanl failure; it means i failed to build the required aggro, but its always preferable to a dead healer. taunting off a tank is something i strive not to do unless required by the encounter, but even then i've had pug co-tanks with rune of blood immediatly taunt back after a taunt off them on saurfang, and ones who usd them on CD tomaintin aggro. If you where attempting to taunt swap with taunts, but your cotank was just spamming his taunts you would also have aggro problems like you mentioned. WhoTaunted is my current favourite addon for tracking raid wide taunts it prints a message to the chan channel of your choice listing who taunted what with which ability, and the effect (misses. immunes, etc), it's real good for seeing if your co tanks are taunting irresponsibly.