I am using the Barebox bootloader to boot into an embedded system.
I am wondering why some SD-card of mine shows up as 2 devices
And sometimes another SD card it shows only as 1 device
Likely the first sd card is partitioned. /dev/disk0 is your sd card and /dev/disk0.0 would be one of the partitions on that card. You can analyze the partitioning scheme further using your favorite partitioning program, popular ones are fdisk for terminal and gparted for gui.