Upload & Sell: Off
I hadn't paid attention, but I guess it only deletes the file from the card you've chosen to use for playback. (I rarely delete individual shots in camera. Once all are on a hard drive and backed up, I format both cards in the camera.)
You might get more views (and maybe someone knows a way to set it up to do what you want) if you change your subject to "deleting from both cards in 1D Mk IV".