I wasn't clear: the second SD card is to play with more than one OS or to be able to quickly swap a working distro in case the first one goes wrong (dd to a file is of course the mandatory way to go but having the second SD card plug-and-play is more convenient IMO:no fdisk involved).
Ah, alright. That makes sense then. I interpreted it as you suggesting to dd the first sd card to a second sd card which would then become your backup.
I wasn't clear: the second SD card is to play with more than one OS or to be able to quickly swap a working distro in case the first one goes wrong (dd to a file is of course the mandatory way to go but having the second SD card plug-and-play is more convenient IMO:no fdisk involved).