[...] it never dawned on me that you could have more than one signature in a book [...]
a double spine -
besides, me stupid foreigner never guessed, that a 'signature' could be anything other than an autograph and now knowing better:
This is about the same way, that I did it back in the 90s, when I was pretending to be a writer.
I wanted my 'books' to have a real backspine, so you could read which book it is, when it's on the shelf. On an unprofessional, yet dirty level, my solution was similar to yours: