I believe that all the books that are credited to Andre Norton are entirely written by her.

In my opinion BROTHERS TO SHADOWS does contain what are the usual Norton plot devices and seems to have the same 'voice' as most of her recent novels. However, maybe you have noted an intentional difference in the styles between the chapters 'by' the two shadows.

Perhaps if you had been reading some of the co-authored books, you noticed a difference. In most of those, I believe that the co-author writes most of the book, with Andre Norton providing important editing and advice. However, the co-author is the one getting the re-write requests from the publisher (and TOR usually asks for lots!).