I believe there is a duplex property for printer objects, as far as how to do it though, I'm in the same boat as you having to research.

Same as Adam, I don't know about Duplex printing either, but I did find this online.

'SYNTAX
'expression.PrintOut (Background, Append, Range, OutputFileName, From, To, Item, Copies,
'Pages, PageType, PrintToFile, Collate, FileName, ActivePrinterMacGX, ManualDuplexPrint,
'PrintZoomColumn, PrintZoomRow, PrintZoomPaperWidth, PrintZoomPaperHeight)
    
     Dim objWord As Object
     Dim objDoc As Object
    
     Set objWord = CreateObject("Word.Application")
     objWord.Visible = True
     Set objDoc = objWord.Documents.Add()
    
     objWord.PrintOut , , , , , , , , , , , , "C:\Documents\YOURFILENAME.docx"

     objWord.Quit