Yes, I have a T5820 and i added a RTX3090 (3x 8PIN). How the Power cables are single rail, at a first moment i bought in amazon this cable <COMeap GPU VGA PCIe 8 Pin Female to Dual 2X 8 Pin (6+2) Male PCI Express Power Adapter Braided Y-Splitter Extension Cable 9-inch(23cm)> and i transformed one 8 pin plug in 2 8 pins plugs. That is ok and the RTX3090 worked fine. Later, i was seeing the power supply have a empty power connection (the chassi is the same of 7820, and the empty power connection output is to 2º processor in 7820 Workstations), then i bought in ebay this cable <10pin to 8+6pin Power Cable for DELL Precision 5820 and GPU 50cm> and added to empty power plug output in the power supply connection board and i removed the old adapter (from amazon) and it is working fine too (and with a best visual and better cable organization).
  I have a Precision 5820 desktop with 950W power supply and have the potential to install an AMD RX 6900XT GPU. It seems that the GPU needs 3 x 8 pin power connections when there are only 2 currently available from the PSU. The recommended PSU wattage is 850W so the current PSU should be fine. In which case could I use a cable to split one of the single 8pin outputs from the PSU into 2 x 8pin for a total of 3?  
Precision 5820 + RX 6900XT