A family of Microsoft word processing software products for creating web, email, and print documents.
Install a second copy of the printer driver that has it's default set to print double sided and then use a macro in the document so that when the Print command is given, it will print to that printer.
See the article "Changing the selected (current) printer in Word without changing the system default printer” at:
http://www.word.mvps.org/FAQs/MacrosVBA/ChangeCurPrinter.htm