This question came up in the PS Forum, IIRC, and I was not sure.
I have seen threads edited (after replies, etc.), branched, deleted, locked, moved, but cannot recall seeing two, or more threads combined. Not sure what the format would be, as I can well imagine that in either Threaded, or Flattened View, it would be very confusing.
All that I have ever seen is a link to another trhead(s) posted into another.
I have seen this merging done in other (non-Adobe) forums I frequent. I have no idea how they do it, but the net result is one forum with posts odered by date. Some pruning is almost always needed to make the combined thread look tidy but, unlike here, when a mod deletes a message, the mod leaves a note explaining the reason for the deletion.
Yes this is possible sometimes if you happen to be on the page. For example, you might have two or three windows of the forums open and one of the windows happen to be that you want to edit. I have managed to do this because I was on the page when somebody replied after my post.
For an example, look at this link: <http://forums.adobe.com/thread/952530> where message number 4 is posted some 30 minutes after my message number 5. The reason being, JW edited his message after my post.