at first: octane is absolutely breathtaking! thank you so much!

I'm a Octane-newbie so sorry if my questions are too stupid. today i add a second gtx 260 and now it seems that every second frame is not correclty rendered anymore. if i disable the second card everything is fine. here is a result where you can see what happens:
http://www.vimeo.com/12378553
another question: is it possible to make a HDRI-MAP invisible while illuminating the scene with it. i need a black clean background.
thanks in advance and sorry for my horrible lousy english!
Oli