Motherboard manufacturers sometimes make big mistakes with mobo layout.
Example with X99 and X299 no one had idea to allow customers benefits of CPU with 28 PCI-E lanes.
That mean to connect x4 slot one or two to CPU.
If customer use x16 slot no matter how many lanes device use speed of GPU drop on x8, because 16+16 is 32 and CPU have only 28.
But if you have 28 lanes and GPU use 16, you have option to install M.2 Gen 3 to use CPU lanes, and one x4 slots where customers could use Intel 750 SSD or PCI-E SSD example and speed of GPU will not drop. With X299 Gen3 PCH allow better posibilites, but with X99 Gen 2, you could use PCH eventually for sound card but several PCI-E lanes are free and big slots can't be used and no such advance motherboard to recognize how many lanes your device installed in x16 slots use. She immediately recoznie as all 16 or 8 PCI-E lanes even if you put sound card there or x4 USB 3.1 card.
My RVE10 is only mobo I think with x1 and x4 slot in same time.
And in theory have enough PCI-E lanes to M.2 x4 CPU, PCI-E SSD x4 CPU, GPU x16 and Sound card x1 PCH work in same time on 28 lanes CPU.
I didn't tried and can't say is it possible, but could be build.