We have had a couple authors who have been incredibly prolific in certain boards (off topic, B&U, G&C) whose threads were merged and the author PMed about it. What Neroon said, and:
Similarly, if two threads by the same author are posted in the intro board, the system merges the threads.
If, in your case, the originating author wasn't the same and the subjects of the threads were completely different, I'm not sure what it'd be about. You'd have to PM me specifics for that one.