Per Corsair that cable is designed to convert two 8pin PCIe type 4 ports on the power supply to provide a single 12VHPWR output to a graphics card. Some of the new 9000 series AMD cards use 12VHPWR however it's unlikely that your 7800XT does. If you look at your card it probably has two 8pin PCIe ports.
https://www.corsair.com/us/en/p/pc-components-accessories/CP-8920284/600w-pcie-5-0-12v-2x6-type-4-ps...
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