Well call it bad optimization/coding then, I bet there needs to be more done to make multi-GPU's work properly.
I noticed that option too but if you do a test by disabling one of your GPU's and try it that way and it works, then you know.
I can tell you it runs smooth as silk on my single AMD 7970 GHZ Edition card.
oh trust me it won't be the only place, if you accept the quest to protect the halls of remembrance and put a flame tower as one of your traps it does the same thing
I'll let you know when I get to this location/quest in the game, just started and only level 8 so far, but will be moving forward soon.