On (eg) Testy, post a new chapter, or edit a chapter, to add (eg) testy2 as a chapter co-creator
Make sure this is the only chapter of the work that testy2 is co-creator on
Log in as testy 2 and navigate to the Edit Chapter page for the chapter
Select "Remove Me As Chapter Co-Creator"
testy2 is removed as co-creator of that chapter, but retains co-creator access to the work as a whole
If a co-creator who was added only to one or more chapters (and not as co-creator of the whole work) removes themselves from the last chapter for which they were credited, it should also remove them as a co-creator of the work and remove all associated edit permissions etc.