For this it's best to go to principles, as I do not believe there is explicit guidance in the ISOs (in regulated industries that can be a whole different discussion).
We all make mistakes, and competent checking to capture this is as important as competent authoring. (Although the authoring must be good enough that there aren't so many mistakes that the checker "can't see the wood for the trees".) As we are all human we need one person to write and the other person to check. Otherwise you aren't really doing a check.
It's useful here to break out review and approval:
- A review examines the document from a content perspective. It is performed by subject matter experts who are able to determine if the information is complete, clear and accurate.
- An approval considers the document more from an organizational perspective; that it is appropriate to implement and its use will be supported and enforced beginning on the document's effective date.
I can easily see times when the author and the approver is the same person. The person who wrote a document is also the person to approve it from an organizational perspective (again care for this should be applied in regulated environments like pharmaceuticals). But a separate reviewer should exist to ensure completeness, clarity and accuracy.
Or you can have the author as one person and the reviewer/approver as a second.