You dont.
At least not the way you want it to work. There are two options:
Play with the contents header. Something like 'x-replace-mixed' and then ship out the two html files
after eachother (thus no fork). This does not work on all browsers
Play with some javascript DHTML, to remove the flash when the rest is loaded ok. (no forking again). This will mean that the browser does not recieve a full page and there can be problems rendering it.