According to Dell, the T7600 has an optional second set of PCIe x16 slots above the CPUs that are available only when the second CPU is installed.
I have a second CPU installed (and I know its working), and yet any card I install in these particular PCIe slots are not recognised - move the card to one of the slots below the CPU and the card becomes recognised.
Is there anything I need to do specifically to enable these PCIe slots above and beyond installing a second CPU?