You are not supposed to power the GPU like that. You should use two separate cables from the supply. The other connector of the same cable is intended to “daisy chain” low power cards.
It will probably work anyway, but better safe than sorry.
Edit: I think it’s needed because:
The power supply might have separate circuits for separate cables and might not be able to supply all the power needed by the GPU through just one
The cable might not be rated to have that much power flow through and might overheat and melt over time
If you could just fork the cable into two why would they put two connectors on the GPU, it’s not like they have different voltages, they are literally daisy chained