In general, you can’t file a second extension. See below for exceptions however.
Once upon a time you could request a second extension (Form 2688) until mid-October if you couldn't file by the first extension's mid-August deadline.
In 2005, the IRS eliminated the second extension and updated the first extension deadline (Form 4868) to October 15.
Exceptions that allow for a second extension include:
- Rare cases involving recent and widespread natural disasters
- Some members of the military as outlined in IRS Publication 3 - Armed Forces' Tax Guide
- Some taxpayers living outside of the U.S. as outlined in IRS Publication 54
For tax year 2019, the extended filing deadline is October 15, 2020.