Borrowing this idea from Aerospace, where you can use "main" to target the main display, and "secondary" to refer to the other display when there are only two of them connected.

Likewise, it can be sometimes confusing to find exactly the names of displays when you want to quickly write a one-off command, as I do all the time when I plug in my laptop into different displays. It would be convenient to use "main" and "secondary" as targets.

For the time being, I'm making do with the following command line to move the secondary display to the bottom of my main display:

betterdisplaycli perform "--originalName=$(osascript -e 'tell application "System Events" to display name of desktop 2')" "--targetOriginalName=$(osascript -e 'tell application "System Events" to display name of desktop 1')" --moveTo=bottom