> I need to figure out if PLX switching is required,
> or if the lanes can simply be separated via routing.

There's some info on PCIe bifurcation/trifurcation on the Talos II (Lite):

https://www.phoronix.com/forums/forum/hardware/processors-memory/1025053-raptor-launching-talos-ii-lite-power9-computer-system-at-a-lower-cost?p=1025214#post1025214
https://www.phoronix.com/forums/forum/hardware/processors-memory/1025053-raptor-launching-talos-ii-lite-power9-computer-system-at-a-lower-cost?p=1025310#post1025310
https://www.phoronix.com/forums/forum/hardware/processors-memory/1025053-raptor-launching-talos-ii-lite-power9-computer-system-at-a-lower-cost?p=1025326#post1025326
http://tenfourfox.blogspot.com/2018/05/a-little-talos-of-your-very-own.html