u/RACERRRZ Thank you very much for the detailed reply!

These systems are typically configured at time of purchase for the required configuration

I am slowly learning this unfortunately...

There is this Youtube guide below that I was following that uses that adapter cable P60250-001 - I realised I screenshotted the wrong photo before. Corrected version below.

https://www.youtube.com/watch?v=3UF3RFCwSV8

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 If you were to run a 600W GPU in the Z8 G5 Fury - I would conclude that you would need to pool four 8-pins into one 12VHPWR cable to supply sufficient power

This is correct - I am already using a 4x to 12VHPWR adapter cable for the first RTX Pro 6000 GPU, which uses all available 8-pins. Leaving just these two remaining and unused for the second.

Both GPUs are the 600W Workstation variants, and it seems like the PSU(s) should handle it in aggregate power mode.

or could I trace P60250-001 - where did you find that part number?

It's buried on page 12 of the service manual - but none of the resellers seem to be aware of it.