Ok, I see what you are talking about. But, I really don't know what your end goal is.
AutoNew fires before document_new. You can see that your self by putting different message boxes in each.
Try this:
Code:
Sub AutoNew()
Dim lngNewRandNumber
Dim oDoc As Document
MsgBox "AN"
Randomize
lngNewRandNumber = Int((1000 * Rnd) + 1)
MsgBox ThisDocument.CustomDocumentProperties("RandomNumber").Value
ThisDocument.CustomDocumentProperties("RandomNumber").Value = lngNewRandNumber
Set oDoc = ActiveDocument.AttachedTemplate.OpenAsDocument
oDoc.CustomDocumentProperties("RandomNumber").Value = lngNewRandNumber
oDoc.Close wdSaveChanges
ActiveDocument.CustomDocumentProperties("RandomNumber").Value = lngNewRandNumber
MsgBox ThisDocument.CustomDocumentProperties("RandomNumber").Value
End Sub