It's perfectly fine if similar questions are asked on two different SE sites, where the questions are on-topic.
Answering the question with a link to the other question, or to an answer for another question is not a good practice. An answer with a link is generally considered not an answer, on SE sites. If the linked page is not reachable for any reason, the answer would not be helpful at all; in the case the link is to a question on another SE site, the question could be closed because, for example, is a duplicate of another question. Answers should be useful independently from external websites that could be later closed.
Answering to a question on Drupal Answers with a link to another question on Stack Overflow, especially if the question on Stack Overflow has already an accepted answer, means to "push" the user to go to Stack Overflow, instead of using Drupal Answers, and that is not really what we want to happen.
If both the questions on Drupal Answers and Stack Overflow are asked from the same user, who asked the question on the one site before anybody could answer to the question asked in the other site, then I think that is a different case. In such cases, I report on Stack Overflow that the asked question is being asked before anybody could answer the same question the user asked on Drupal Answers.