I have it in DX11 with MsAA at 8x, but it seems that some things aren't processed for AA while others are.
I tried DX9 with 2x6950, but I had the same horrendous frame stutter (30+ fps for about a half of a second, pause for half of a second...very choppy).
I seem to be playing at 30-120 fps, with very rare dips below 60 fps.
Anyway, looks like I made the right call to play this game before my 2013 rig. My 2011 rig handles this fine and it's not anything exception, especially considering the hardware requirements.
Try it with one card and see what happens.