I like the idea of merging posts, especially if a separator line is used.
But it would be a good idea to add a "x minutes ago" timestamp above each segment, even if the username is not added. There are occasions when the time at which information was posted becomes important.
Another thing to consider is what happens to likes on the first post when the second post is added. And of course there's the issue of max post length that Kulga mentioned...
Perhaps instead of merging the posts, you could just tweak the post formatting of both posts so there's less padding above and below the divider line?
Toby The ability to move posts between discussions is something we want to get to eventually.
Also the ability to split threads and merge one thread into another (which I think has been mentioned perviously elsewhere) could be done as special types of post moving.