While I'm not real sure about this... I think that PCI slots are numbered to the computer. Meaning that there are separate lines to each slot. In other words if you tried this there would be two cards on slot 1 and they would conflict with each other, instead of showing up as slot 1 and slot 2. Another clue that leads me to believe that is true is the picture of the PCI risers you found. If you look closely you will see that any of the risers with more than one slot on them have a set of wire that connects to the rest of the PCI slots on the board. These are the special lines to each PCI slot that I am referring to.

So I would say that you can't add more slots. These PCI risers you found simply relocate the PCI boards in the machine.

I could be incorrect.