You can file Form 8379 (injured spouse form) afterwards by itself.
Here is the link to the form: https://www.irs.gov/pub/irs-pdf/i8379.pdf
You will see a chart on the first page that says "Where To File". It states that if you file Form 8379 by itself after you filed your original joint return electronically, then mail Form 8379 to the Internal Revenue Service Center for the area where you live.
