It's perfectly fine to use two cables with 6+2 pin connectors. In fact with a high draw GPU, I prefer using two cables as it reduces the load on each cable. That said, a quality PSU such as the G2 650W has cables capable of taking the full power draw. My 1080Ti is hooked up with 2 6+2 cables on a G2 850W. The only drawback to using more cables is that you then have to organise/tidy the extra cable.
Seasonic actually forbids using one cable from PSU split to two power connectors of graphics card.
And voltage loss from too high current per cable/wire was apparently behind this reboot when under heavy GPU load problem:
https://forums.overclockers.co.uk/threads/ocp.18787566/
You have to remember that using fewer than intended wires for same current/power causes not only higher voltage loss to 12V leading to graphics card seeing lower voltage...
But also higher voltage loss in ground wires rising ground more above actual zero.
And that might be bigger problem than fluctuation of 12V input voltage:
8-pin PCI-e connector with its higher current/power rating than 6 pin connector actually doesn't have any more 12V wires, just more ground wires.