do a search - i seem to remember a similar question being asked and answered before… the background of a swf can be transparent… hope this is what your after…
btw, just out of interest, why do they have to be separate files?
Hey you can load new movie full transparent on running one.
Just dont put any background in the second movie.
Your graphics in second movie will be visible directly on the background of frist movie.
Try it.
(it will better if you put the stage size of both of both the movies same)