I think you may be misunderstanding the requirements.

From what I am reading, I believe the OP means 2 per sheet, front to back spiral bound. If this is the case, just use a separate odd/even header/footer, and let the printer duplex the document.

And, if the OP really means 2 per page (i.e. 4 pages per sheet - 2 front and 2 back for a bound document), you would still create the document the same way. Just let the printer handle the imposition of the document. (Alternatively, you could create a PDF- certain versions of Acrobat have an option that can impose the booklet depending on the printer, but you would still set up the Word document the same way)

Either way, you should let the printer handle the details of how to print and bind the document- that's what they do. I'd check with them first to see how they need you to set up your document, in case there are any special requirements.