preload external swf, then load another external swf on top of it

Hi,

Normally I do get by with a lot of R&D, but for some reason am stumped and really pressed for time! Tried some other forums, but haven't got a solution thus far. OK, here goes...

An interactive file (say main.swf) is already ready to go.
I now need to make this play over a background (say bg.swf) which is a fairly heavy file.

What I'm trying to do it this:

Open a new flash document (say "index.fla")- on the first frame, call "bg.swf" by using "loadMovie('bg.swf', 2)" [where 2 is the level number]
and then calling "main.swf" by using "loadMovie('main.swf' 4)" [where 4 is the level number]

Now the problem I am facing is this:

"bg.swf" is quite a heavy file so takes quite some time to load, whereas "main.swf" is much lighter and loads before "bg.swf" buffers fully.

What I want to achieve is this:

Create a new swf file (say "index.swf"), making sure that "main.swf" does not load until "bg.swf" loads completely under it.
How is this achievable?

Note: I cannot make any edits to "main.swf" (which is like a slideshow) file and it is rendered such that it starts the slideshow after 5 seconds.