Duplexing (printing on both sides of a sheet) is different from printing "2 pages per sheet" (two pages on one side of a sheet). Duplexing is controlled by your printer Properties. On the Print tab of the File backstage, beneath
Printer click on "Printer Properties." This will open a dialog generated by your specific printer driver. This is not part of Word but will vary depending on the printer, so we can't give specific instructions on where to find the duplexing
setting, but it will be somewhere in the Properties if your printer has the ability to duplex automatically.
Additionally, if your printer is capable of duplexing automatically, then the dropdown list under
Settings that by default says "Print One Sided" will also offer options for duplexing; mine has "Print on Both Sides, Flip pages on long edge" and "Print on Both Sides, Flip pages on short edge."
If your printer is not capable of duplexing automatically, then yes, you must duplex manually. Under
Settings: Print One Sided, the dropdown list will offer only "Manuall Print on Both Sides" as an alternative to "Print One Sided."