As far as I know, that is not possible because the files in the Nintendo folder are encrypted with a console specific key.
So you can't share the same Nintendo folder with 2 Switches.
You can rename the Nintendo folder for each Switch every time you swap the cards.
Like :
1. take the card out of Switch #1 and put in a PC
2. Rename the Nintendo folder to Nintendo_Switch1
3. Rename the Nintendo_Switch2 to Nintendo
4. put the sd card in the second Switch.
AFAIK a Switch only adds the Nintendo folder to the SD card the first time, it doesn't format the card.
But be sure to make a backup of the SD card when creating the Nintendo folder on the second Switch.
After that is done, you can always copy the Nintendo folder for the first Switch back onto the SD card.
If you use a Mac, note the archive flags. Hekate has a utility to fix wrong folder archive bits.
Also, you can't use a game installed on the first Switch on the SD card with the second Switch and
your save files are saved in the internal memory.
