The IRS states: If you e-file your return but choose not to report each transaction on a separate row on the electronic return, you must mail the form 8949 to the IRS. You need to mail form 8949 when you choose the summary transaction method.
If only one of the parts applies (that is, you only have short term gains, or you only have long term gains), you only print the part (part 1 or part 2) that applies.
The following is directly from the IRS's 2016 Instructions for Form 8949, (Page 3, Box F): https://www.irs.gov/pub/irs-pdf/i8949.pdf:
''You don't need to complete and file an entire copy of Form 8949 (Part I and Part II) if you can check a single box to describe all your transactions. In that case, complete and file either Part I or Part II and check the box that describes the transactions. Otherwise, complete a separate Part I or Part II for each category of your transactions''
Page 1 of Form 8949 is for short-term transactions, and page 2 is for long-term transactions. If you have no short-term transactions, then page 1 is inactive. Essentially each page functions as a separate form.
