CarpNCod

Scratch that. I just re-read the initial post and saw these GPUs are on separate nodes. The 1660ti does indicate it is only getting 4 of 16 lanes though, which why I assumed (incorrectly) they were on the same node.

I would guess that this is because the 1050ti is in PCI bus 1, device 0 and is probably using 8 of the 16 PCI lanes, whereas your 1060 ti is in PCI bus 1, device 1 and is only using 4 of the 16 PCI lanes. You will want to check the documentation for your mobo. Most AMD boards provide 16 lanes for one GPU, and if you add a second, split them 8/4.