Settings for printing both sides of a page are specific to your own printer. Go to Print and choose options. You should see specific choices there if your printer has those options.
Record a macro as you choose those options and then view the resultant code to automate the needed steps.
However, some printers have a duplexing option that will allow this to be done a lot smarter, and without having to resort to printing the odd pages and even pages, but this would be accessed thru the printer driver options, not Excel. If you haven't got a posh printer, then the above is the only way...