I am starting a forum (for mental and other health issues) that some members (because of multiple health symptoms) can post a new topic in more than one forum. I was wondering how easy would be to give the option to the users to list the topic in more than one forum and get input from both? Is this something difficult to get done?

You could create only one forum and configure tags system properly for your needs.

After forum creation configure rules with html to link each tags.

Into this way you can find topics with some tags but the system currently not provide counter of topics/replies and last post for tags if you don't need it this is a good solution.

The better way is to build a custom query with all tags for this forum and format for result instead of rules of forum.

Post it in one forum, after you are done move it to your other forum. 

The topic will  show in both forums. The first one you posted is just forwarded to your other forum.

Don't choose to delete the "moved link" in 14 days, so it stays there indefinitely.