I think if you have 1st and smaller M.2 that would be used to install OS with no question. 2nd is left larger for everything else. Either way, it's still recommended to reinstall Win on the new PC to have a fresh install. So during that process we can format the disk where the unwanted OS is and install it to the disk where we actually want the OS to be.PCS would by default add the operating system to the fastest drive...in identical drives, it would be added to the smaller of the two. However, it doesn't always happen that way in practice...there have been builds where it's been put on the bigger drive so a note is worthwhile/
I think if you have 1st and smaller M.2 that would be used to install OS with no question. 2nd is left larger for everything else. Either way, it's still recommended to reinstall Win on the new PC to have a fresh install. So during that process we can format the disk where the unwanted OS is and install it to the disk where we actually want the OS to be.