If you choose to use the RAW to card 1/JPEG to card 2 option, but shoot RAW only, the RAW is recorded to both cards--just as if you had chosen the backup option.
I would have expected the camera to write the RAW file to only card 1 and leave card 2 alone.
Why does it write to both? Not a big deal, but does that mean more time is used to write data to both cards than if writing to only card 1?
I never noticed this before because previously I almost always chose the "overflow" option and also almost always shot RAW plus Jpeg. As expected, the camera wrote both formats to card 1. (I almost never fill up card 1 before being able to download and clear the card).
For those times when I did choose RAW to card 1/JPEG to card 2, the behavior was as expected--each card received the specified format.
I recently made a decision to shoot RAW only unless I had a specific reason to also shoot JPEG. And then I noticed I was getting RAW recorded to both cards even though I had still left the card option to 1/RAW, 2/JPEG.
#2. "RE: Unexpected card behavior on D7000" In response to Reply # 0
Perhaps the reasoning is that the designers thought of RAW+JPEG as a means of saving a backup JPEG along with the RAW. (That's the way the manual describes it.) If you then elect not to shoot the JPEG, does that mean you don't want a backup at all or that you want the backup to be RAW? There is no obvious answer to that in my mind.
As you say, it's no big deal since you can get either behavior (RAW backup to second card or no backup at all) with the right combination of "Role played by card in slot 2" and "Image quality" settings.