Maybe you can use Flash to get the desired effect? It shouldn't be too hard to put two movies back to back with that, even if you need to run some sort of conditional for the second movie.
Other than that, there isn't any PHP function that will help you to get this information. There may be a shell command I am not aware of that will allow you to get this, in which case you would only need to run the command and get back the output using system [us2.php.net], or any of the similar functions.
I hope this has provided some kind of plausible solution :)
Oh, why don't you just edit the movies and put them together as one? Another possible idea ;)