Miller had been working on the sequel (for many, many years apparently) and more or less finished it when his bout of depression got quite serious and he couldn't face working on it. He then contacted Terry Bisson and asked him to help straighten the book out. During this time Miller killed himself. Bisson and the publishers decided to complete the book with as minimal requirements as possible and then put it out.

IIRC, Miller's manuscript ran to 600 pages and was quite flabby, so Bisson acted almost more as an editor than a co-writer, paring the book down and only writing new elements where absolutely necessary. Some of his written material was approved by Miller before his death, some of it not.

So it's different to the Sanderson situation as Miller himself requested Bisson's assistance and was working with him on the book when he died. Bisson himself claimed no cover credit for the book, saying his work was pretty much minimal and 99% of the text in the book is Miller's work.