In this case, one question is more specific than the other, as the question on Signal Processing ask how to do it with Matlab, while the question on Stack Overflow doesn't ask how to do it with Matlab.

I think the question could be asked in two different sites, if the question is made specific for the site where the question is being asked. For example, if somebody asks the exact same question on Drupal Answers, and Stack Overflow, where Drupal questions are on-topic, that would not be acceptable.

In this case, one question is more specific than the other, as the question on Signal Processing asks how to do it with Matlab, while the question on Stack Overflow doesn't ask how to do it with Matlab.

I think the question could be asked in two different sites, if the question is made specific for the site where the question is being asked. For example, if somebody asks the exact same question on Drupal Answers, and Stack Overflow, where Drupal questions are on-topic, that would not be acceptable.