Pcie slot #1 is x16/x8/x4. If you put anything into pcie slot #2, both slots will run x8/x8, same as sli or crossfire.
As others have said, there's no gains to be had with using nvme. With a newer motherboard, the pcie lanes are from the cpu direct, or pass through the chipset PCH, just a single dmi bridge. Very little loss if any. Using an adapter, you almost always lose a bunch of bandwidth and the nvme will be effectively running at x2, not x4, making it almost exactly the same performance as a standard sata drive.
First is the interface differences, then the bridge in the adapter, then the PCH.
Sata is cheaper to get realistically the same results.