Some important points on this:
1. The main tables that would have to be *handled* in a merge: wp_posts and wp_comments. It's possible wp_categories and wp_post2cat would we needed (if a blog has categories you wish to preserve), wp_postmeta, and even wp_users. (Note the table prefix may vary.)
2. The difficulty in merging two or more databases/table structures is the unique identifiers for the various components; ID for the posts table, comment_ID for comments, and cat_ID for categories (and let's not forget rel_id in post2cat, meta_id for postmeta and ID for users). These values could be NULL'ed in the dump file and an import would correctly reset them, but you still lose the connection that exists between post<>categories in post2cat and post<>comments in comments (as well as post<>custom field data in the postmeta table, and user<>post in the post table).
3. All this is easy to manage if were talking about a few, or few dozen, posts from the database(s) to be merged. You could import the posts, collect the new post IDs, and modify the other tables in the dump file to point to the correct (i.e. new) one. When it gets to hundreds of posts this becomes difficult.