WritingInPairs is a concept very similar to PairProgramming: it occurs whenever two people collaborate at one computer on an important email or a few difficult paragraphs. It has many similarities, and some differences.
Differences:
WritingInPairs or writing as a group is a good way to communicate. If Person A writes a document and Person B reads it, A gets to understand the topic very well, and B gets to understand it a bit -- probably imperfectly.
But if Persons A and B write the document together --- even if A starts as the expert and B the novice --- then understanding is transferred very effectively.
WritingInPairs is excellent for DispersedWorking -- use NetMeeting (or similar) to collaborate on a document.