There isn't really any simple way to merge sites, as both sites will have used the same IDs for posts and users (1, 2, 3, etc).
But it should be possible by updating the IDs in the site that you're transferring from to ones that are not used in your first site.
Example: your user IDs go up to 100 on the first site and qa_posts goes to 300. You could add eg 150 to the users and 350 to the posts from the second site, then transfer those rows into the first site.
Note also that you will need to add the same numbers to all other references to those IDs, eg adding 150 to qa_posts.userid, qa_posts.lastuserid; and adding 350 to qa_posts.parentid, qa_posts.selchildid. There may be many more I'm forgetting.
Hope that helps!