It depends on whether your printer supports duplex printing. Check your
printer's features to find out. If it does, the setting will be available in
the Print dialog.
You can also do a "manual duplex" job by printing page 1 (or 'Odd pages'),
flipping the paper yourself, then print page 2 (or 'Even pages')... If it's
only 2 pages that might be the simplest approach.