ASUS PRIME Z270-P has 6xPCIe slots. So I use 2 x PCIe -> 2 PCIe splitters.
SuperMicro X10SRL-F has 7xPCIe slots. So I use 3 x PCIe -> 2 PCIe splitters.
I thought so; what ports are the splitters attached to; both on the mobos and on the splitter boards themselves.
I want to try to predict all possible indexes when a splitter is used in any port.
I've already tried to change PCIe indexes in /etc/X11/xorg.conf (Section "Device") in accordance with to lspci, but did not see any changes.
Maybe I did something not right...

Not sure, things get a little weird after 7 GPUs. I want to support up to 14 GPUs to be ready for the new asrock mobo; so I will work on this for next version.