How about automatically merging them? Like this:

The post is untouched, but the black parts aren't shown anymore. Now that I type it, I realize this is probably much more work to do. And even more work if the second post is on a new page.

I can think of a few acceptions:

• When the user posts "Reserved" in his own new topic
• When one of the posts is >20 kB (that leaves some space for later editing, since the post limit is about 64 kB)