Does someone explain how I can install 2 1080ti on one 500W? I didnt see another double 8 pin wire to connect.
you are asking for trouble even running one 1080ti. Easiest way to kill your GPU run it on a PSU that is too small to run it. You would have to be running it at around 60% and it MIGHT be ok.
Unless it is a really really good brand gold or plat you are lucky it hasnt died already.