I have a development local blog and a remote production blog. The weblogs and posts are identical except for the URL references within them (e.g. for posts and images).
If I edit a post on one of them then change to the other weblog via the drop-down list, change the URLs to the correct ones then save the edited post to the newly-selected weblog, the post appears as a new post and not as an edit of the existing post.
If I refresh the blog after changing blogs in the main window, then open a post to edit it, it posts as an edit and not as a new post, which is expected behaviour. I don't think you can refresh the post list when changing blogs by using the drop-down list.
In other words, it doesn't work to edit a post locally then post the edits to the equivalent post on a remote blog, just by changing blogs with the drop-down list and changing URLs.
This makes it hard to keep a local and remote blog synchronised during an editing session. It also makes a mess of the remote blog as you have to delete offending duplicate posts