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I want to upgrade my gtx 1070 too an rx 6600xt with has 2 8 pin plugs so should I use an 8pin too 2x 8 pin conector .everything else is 0k the power supply and there is no bottleneck.
If you use an adapter, the current which is meant to flow over two cables will flow over one. in your case with a 6600XT oder GTX1070 this is not an issue because the power consumption of this card is relatively low. But generally, you want to use as few such adapters as possible to minimize the currents in your cables.
Note: the language of PCGHX is german - so if you can do so, please try speaking german.