You can use media_playing(handle) to return the current frame of your stream, then pause it once it reached its target frame and then start the next one. Something like this should work:

Code:
...
var handle_intro_played = media_play("demo.ogg",NULL,100);			// play your stream
while(media_playing(handle_intro_played) < target_frame_number) { wait(1); }	// keep playing until target_frame_number has been reached
media_pause(handle_intro_played);						// pause the first stream
var handle_next_mediastream = media_play("demo2.ogg",NULL,100);			// start next one
...

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