Cycles Multi GPU incorrectly rendered, is this a bug?

I recently bought a couple of GTX 570 which are not sli configured and I have a rendering issue which seems to occur when I select the use 2x GTX570(multi gpu) option in preferences.

I have tested each card separately by selecting only one in the options and they render the scene correctly this issue only occurs when multi gpu is selected.

After lots of testing and thinking possibly memory related as they are only 1.28Gb I think I have found the issue.
It seems it’s to do with the environment settings more specifically the option to sample as lamp when using multi gpu. I will show the before pictures and after unticking the option. If I click the option to switch it on and off enough times sometimes it will render correctly with this option enabled but this is not reliable for rendering the animation.
(low passes just to show the issue)

Is this a bug?