I've never had a problem running two FX1800 cards in the xw6400, xw6600, xw8400, Z620 or Z600. I have not tried that in a Z800 but cannot imagine it would not work.
You may have a bad card. Another obscure cause is if someone messed with the BIOS and turned off the slot in the Security settings. Also check to ensure that the slot is set to default PCIe settings. I'd bet it is a bad card and try another junker PCIe card first.
