Generally with duplex printing, the printer need to be switched into duplex mode, odd number pages are printed on one side of the paper, even number pages are printed on the other side.
Since you probably don't want all printing done double sided, I would think that the easiest approach would be to set up another printer in your printers folder, which points to the same printer but is configured for duplexing. This is much easier than trying to do this through Access.