I see Drupal questions in Stack Overflow (SO), sometimes they are transferred to Drupal Answers (DA) and sometimes not. Some questions by new site users might get better responses on DA, but if the user is not aware of DA then they aren't exposed here. Is it appropriate to suggest trying on DA as well?
Both sites are part of the StackExchange network, StackOverflow is the original site that started it off. There are now many sites that are part of the network, including this one.
This site is focused on Drupal, so it would make sense to have Drupal questions here. But it is policy that where there is overlap between sites (and this seems to happen a lot), and a question is equally on-topic on any of those sites, then it can be asked on whichever site seems the most relevant. Questions can be migrated afterward if the community deems it appropriate, or the asker requests the migration.
Given that StackOverflow is about coding questions and answers, questions involving Drupal where code is concerned are equally on-topic here and there.
The upside to the Drupal site is the focus on Drupal, so you may be more likely to get an answer you need here. On the other hand, StackOverflow gets much more traffic, so a question there is more likely to be answered quickly.
Is it appropriate to suggest trying on DA as well?
In my experience, suggesting to try on Drupal Answers as well has only the effect that the OP asks the same question on Drupal Answers without flagging the question for moderation attention to move it on Drupal Answers.
If you are going to suggest to try on Drupal Answers, then suggest to flag the question to be migrated, or to ask the next question on Drupal Answers. As long as the users understand they should not ask the same question in both the sites, the suggestion is fine.
That said, programming questions about Drupal are still on-topic for Stack Overflow; the OP could not know Drupal Answers, or could prefer asking the question on Stack Overflow because he asks most of the questions there. There is nothing wrong with this, and I would rather avoid asking a user to ask questions on DA, if he doesn't ask much questions about Drupal.