look for 16 awg 18 awg is dangerous imo
This. And even then I'd be skeptical. 8-pins can deliver 150 watts (6 pin can do 75 watts) or 12.5 amps each so running twice that off of one cable is not a good idea.
The CONNECTOR used for 8-pin PCI-E is specified at 8 amps per pin - multiply times 3 (as only 3 leads are used for +12v) = 24 amps x 12 volts = 288 watts.
PCI-E spec is VERY conservative on the connector.
Running an 8-pin plus a 6-pin should be viable if you mind the wire gauge, on a high-quality power supply.
However DEFINITELY make sure all of the wires used are at LEAST 16 AWG.
The other question is WHICH EVGA power supply are you using? They DO make some
that will handle 4 x GTX 1080 ti in a rig with ONE power supply (G2 1300 for probably the best example) - and THAT power supply has enough connectors by default.
If you think you need to modify the connectors to "make them fit" you are probably trying to put them in wrong.
No, 1080 Ti's require 2 x 8 pins and the G2 1300W comes with 6 VGA ports on the PSU and 2 x 6+2 + 6 cables and 4 x 6+2 cables
That's 5 x 8 pins total (they might as well come with only 8 pin single cables) without converters meaning it can only handle exactly 3 pieces of 1080 Ti's.