This post at the Republic of Gamers forum may help:
https://rog.asus.com/forum/showthread.php?115203-Crosshair-VIII-Impact-m-2#post792054
It indicates that according to the motherboard manual (page 1-8) M.2_2 is provided by the CPU and M2_1 is provided by the X570 chipset. According to the post, this is why M.2_2 operates at PCIe 3.0 when a 1st or 2nd Gen (Zen and Zen+) CPU is installed. Those older CPUs only provide PCIe 3.0. With a 3rd Gen Ryzen (Zen2) CPU M.2_2 will operate at PCIe 4.0. In contrast the M.2_1 always operates at PCIe 4.0 regardless of what generation of Ryzen CPU is installed. This is because the X570 chipset provides PCIe 4.0.
So I would say use M.2_2. It's connected directly to the CPU and it will be cooler for your drive.
Also, I always install Windows with only my boot drive installed to make sure the OS is being installed to the correct device. This actually used to be necessary when installing to an SSD with earlier versions of Windows. It's not necessary any more, but I still do it and then install other storage drives after Windows is installed.
I hope this helps.