Strongly advise don't do this.  Sata connectors are designed to safe for up to 4.5 amps. The risers could draw up to 6 amps especially using 1080ti.  Much better to use 2 molex on one string.  Each molex won't have a problem as each can do 11 amps.  The only issue is the wire will get a little warm but not enough to be dangerous.