does wiring matter for the separate PSUs? I have three risers and the attached GPUs being powered by one PSU, and the rest of the computer and 2 GPUs and their riser powered by it.
What Miner are you using? start with at least 2 GPU without risers, 1 GPU to the main PCI-e x16, and the 2nd GPU to the next PCI-e x16, then fire up miner.
Use some easy to use miner like EWBF or Claymore for testing or troubleshooting purposes, check also your PSU power rating 1080ti's are power hungry especially when mining. goodluck and happy mining