See
Individual merge letters or
Merge and Split
Without debugging your macro an obvious problem seems to be at
Code:
Set docSingle = Documents.Add 'create a new document
which will use the normal template. It would be preferable to use your merge document as a template.
Code:
Set docSingle = Documents.Add(Template:=docMultiple.FullName)
You might also bear in mind that a merged document is divided by sections, so you can use the sections to split the document which is rather less complicated than using the pages.