Shog's answer is excellent. I'd just like to articulate briefly why that's the way it should be done on SE. As I see it, there would be basically 2 reasons for wanting a "multiple flag" feature:
- Flags aren't getting handled fast enough
- Flag reasons don't cover all the issues with the post
In the former case, it remains to be defined what fast enough is. After all, if it has not gotten enough attention from high-rep users on the site to get deleted through a low-quality queue, has it really received enough attention that it becomes an issue for the site?
In the latter case, it really doesn't make a lot of difference, since if the post really has multiple issues, the reviewers will see them. There is, as Jenayah said, no difference between what queue they get sent to, and when they arrive there, moderators / LQ reviewers frequently pay little attention to whether the post was flagged as NAA or VLQ unless they see only marginal issues with the post.
To add to what Shog9 has already said, if you are concerned about inexperienced users seeing garbage on the site and thinking that it's the norm, don't just flag. Drop a friendly comment to the user as well mentioning what needs improving in their posts. A complementary link to relevant meta discussions can also be helpful. This can be beneficial in multiple ways:
- It gives the user a heads up on what needs fixing earlier on, so if they are actually committed to producing good material, they have a larger window of opportunity to do so
- In the unlikely event that a reviewer is mystified about the reason for your flag, it gives them clarification
- In the event that the post is edited between your flag and reviewers viewing it to comply with site guidelines, it can alert reviewers to the difference, which in some cases can result in a flag being deemed helpful even though no action was needed.