Uses for SPI Flash

[z4v4l]

you should understand the difference between what is possible (to do with PBP, in this respect) in theory and what is available now. now your (safe) option is to flash the image, containing both uboot and linux, onto eMMC or use what's preinstalled there while use NVMe SSD as an additional storage device, because it is not supported (yet) by FW for the OS booting (still, with some trickery, you can have something even now, read at the end). In future, it may change, NVMHCI may get support in uboot and then direct OS booting from there will be reality, but I don't know if that will happen. and I don't know how exactly installation process will look like then, it depends on how it is implemented, but to answer your question - no, there is no need to have 2 SD cards for the whole installation. it's not needed. installation may vary from an edgy, careless script plainly dd-ing things from a source to a target to a sophisticated installation program with fancy interface and loads of options and questions to you. I assume all provided images have uboot inside, that behaves the same way as the preflashed one, with respect to boot ordering, meaning they give SD the highest priority - this thing lets you flash different linux distributions onto your PBP. Understand, my "live" OS, running from an SD card and doing installation was an example to illustrate. for now, all "installation" is dd-ing the whole image, with uboot and linux in expected places (uboot parts need to be placed at specifical sectors). obviously, with NVMe as an OS installation target, the installation cannot be this easy anymore - you need to put FW in one place and the OS in another. moreover, - places have yet to be figured out, because it's a user choice! see, it's an install program, that we are talking about. maybe someone will make it, in future. my example "live" SD OS just showed that. you need to rely more on what's available now.

if you are ready for messing with that extlinux stuff, if you know how to work with it, the wiki suggests you to try:

It is possible to run the desired OS from NVMe by pointing extlinux on the eMMC to rootfs on the SSD. This requires uboot, the Kernel image, DTB, and extlinux.conf in a /boot partition on the eMMC.