Pankaj, when I saw your comment and thought about looping through the open documents to populate the userbox, I realized I could populate the InputBox with the list of documents and have the user choose them by number:
' Make list of open Documents
For i = 1 To Documents.Count
s = s & vbCr & i & " " & Documents(i).Name
Next i
Set SourceDoc = Documents(Val(InputBox("Choose Source Document: " & s))) ' User enters number of desired doc
Set DestinationDoc = Documents(Val(InputBox("Choose Destination Document: " & s))) ' User enters number of desired doc
Thanks!