My tank and 5th shaman share the same account and I've been debating rolling a DK tank on one of the other shaman's account for this exact reason. The only other reasonable choice would be to buy a 6th account and move the tank over and then you can run whatever shamans you want. Would you rather spend $15 more a month or would you rather have an additional tank to worry about leveling/gearing? If you like diversity then rolling a 2nd tank also gives you the opportunity to try out a new class.

Simpleton, he means that he has 5 shamans that he wants to gear up through instances and because his tank shares the same account as one of his shamans he cannot run this shaman through instances by himself because then he loses the ability to tank for that shaman. Myself and I'm sure many others have this exact problem.