...we also rewrote and footnoted each other's bits as we went along, and rolled up our sleeves to take the first draft to the second (quite a lot of words), and, by the end of it, neither of us was entirely certain who had written what
I can say that I wrote a bit over two thirds of Good Omens.
Initially, I did most of Adam and the Them and Neil did most of the Four Horsemen, and everything else kind of got done by whoever – by the end, large sections were being done by a composite creature called Terryandneil, whoever was actually hitting the keys.
is to either confirm or deny Pratchett's words on authorship by determining which parts of the book were written by Pratchett, and which by Gaiman
Good Omens (1990)
Indeed, the majority of the book is written by Terry Pratchett, and the parts that are supposedly written by Neil Gaiman correspond with those that are about four horsemen, just as Terry Pratchett claimed.