Tank swaps aren't a 'threat race'. If you don't have a DPS on the verge of pulling aggro off of one of you then this is just poor tanking in my opinion, no offense. If the other guy is supposed to take the mob for X stacks then it's part of a tanks job to let him have it until it's his turn to take it back.
Originally Posted by Kalken
"I'll let the dragon hit me in the face, you stab it in the ass."
You may have to change your spec to adjust to the Raid DPS or perhaps put Vigilance on the high DPS/Threat raid members. Normally for me this is on a hunter/boomkin, rogue or fury warrior it depends on their TPS. There is also a technique I recall reading somewhere on this forum which was Vigilance swapping where throughout a fight you will put vigilance on the person with the highest threat atm.
As a druid tank tanking with a DK if my dk taunts off me I can usually keep going my full rotation just not using maul and not pull back (in other words if I were to use maul I would pull agro back).
As a warrior tanking with a druid in my second raid group I can do my full rotation and never pull back (although rage is much more of an issue and unless I have a debuff feeding me rage I can never heroic strike)
But what people are saying is basically true, if your offtank wants to keep DPSing after you taunt, tell him to go cat until he has to tank back. IT IS EXTREMLY BAD tanking to pull back off someone after they taunt because most taunts are for a reason (see: letting debuffs reset)