As I understand, when I post something on Stack Overflow (or other Stack Exchange sites), I have copyright on the contents of the post and I've licensed significant redistribution and re-licensing rights to Stack Overflow.
Stack Overflow then releases that content to other netizens under an open license, but requires attribution in the form of a link to the original answer.
I have occasionally seen users on Stack Overflow who update the 'bio' section of their profile page to say something like 'I hereby release all of my contributions to Stack Overflow, past, present, and future, into the public domain.'.
Since those users do hold copyright, it seems to me this is valid. I.e., anyone using the content can choose to use it under the license offered by Stack Overflow (with attribution required) or as public domain content as offered by the user themself (without attribution required).
Is it valid to use the 'bio' section of your Stack Overflow profile to release your Stack Overflow contributions under a secondary license? Does the answer differ if releasing to the public domain rather than under a secondary license?