This is what someone mentioned on reddit. I am using one cable with two 8-pins. But i think its bullshit because i found nothing confirming this theorie in the internet (Searched for 8 pin two cables vs one):
"As other mentionned, you need to use 2 seperate 8 pins cable (not the 2 connector from the same cable) in order to properly power the GPU. Your 2070 super might not have pulled enough power for you to notice any issues with 1 single cable (I believe it's rated at 215W). But those new cards (3000 series) are power hungry. Usually, 1 x 8 pins can provide around 150W and the PCIe slot provides around 75 - 80W of power. Since the RTX 3080 has been tested to pull up to around 350W under load, you definitely need both 8 pins and a good Power supply to avoid any issues. Your 2070 super was probably OK with 1x8pins + PCIe since it wouldve been pulling 200ish watts in most use case."