I would recommend you print them single-sided as a best practice. But, technically, you can send them in duplex printed as long as each form or schedule is on a separate page. So, for example, you can't have Schedule A on one side of the paper and Schedule B on the other side of the same paper.
Also, be mindful that paper filed returns are entered into the machines by clerks, so printing them on both sides can lead to mistakes in the entry process.
(Note this applies to the IRS but your state could have rules against duplex printing)