you can use one of these

https://www.amazon.de/Grafikkarte-Netzteilkabel-Splitter-Express-Grafikkartenanschluss-8pin-2x8pin/dp/B0762HF1YH/ref=sr_1_3?ie=UTF8&qid=1525009715&sr=8-3&keywords=pci-e+splitter&dpID=51%252BtUKwsPfL&preST=_SY300_QL70_&dpSrc=srch

I use this here for my GTX 1060 since spring 2017, but I never would use for a higher rated GPU with more use of power

https://www.amazon.de/UGREEN-Express-Stromkabel-Grafikkarten-stecker/dp/B01EUSZ0BQ/ref=pd_sim_147_3?_encoding=UTF8&psc=1&refRID=ZMEBQ48Q3DJK6Q1S4C38

or

https://www.amazon.de/StarTech-com-Strom-Express-Grafikkarten-Stromkabel/dp/B007Y8FSMQ/ref=pd_sim_147_8?_encoding=UTF8&psc=1&refRID=7D80V5300SCZE8KVZJ6A

The only thing that I would look for it, don`t put ever-thing on one line, use more lines from the PSU.

But in the end, is better that you are looking for a PSU with 6 PCI-E connectors