Is it valid to post Same Question (related to Drupal) on Drupal Answers and Stack Overflow?
If it's not valid, Can I flag such questions for moderator attention?
It is valid to crosspost the same question if it is going to be tailored for the site where the question is being asked; for example, a user could ask about a PHP library to achieve a Drupal task on Stack Overflow, and ask about a module to achieve the same task on Drupal Answers.
If you see the same exact question to be asked on both the sites, you could flag it for moderation attention, but you need to consider the following points:
In particular, if the questions are a copy-pasta, and asked in the same week, then flag it for moderation attention, using the "other (needs ♦ moderator attention)" reason.
Those users who are asking the same question in two sites normally want a quick answer, and they are normally going to ignore one of the questions once they have an answer on the other site. This means they are engaging two different communities to then ignore any effort in helping.