I've experimented with ATL transitions for Ren'py animation loops and the best way I found of switching seamlessly between two displayables was to have both of them running at the same time on their own layers and then just use show and hide with a fade in and fade out effect.
The scene I was doing was for a rotating skull that would be magically transformed from natural bone to a transparent crystal. Practically every way I tried to do the transistion had some sort of lag/pause or hiccup or forced the new animation to start from the beginning. But when I just ran booth loops at the same time and then hid the top layer with a very slow
Dissolve(2.0)
, it worked perfectly.
It should be easy to do the same with two movies and then use an image button toggle() to switch between them.
As for the audio, I don't know if you followed the discussion we were having in the Dev forum about syncing sex sounds with a movie - but the easiest way by far is just to add the sound track to the movie itself (by whatever video editor you use). But I get what you are trying to do... have two movies running (I am thinking they are two angles of the same scene maybe?) and you want the audio to be in sync, no matter which movie is being shown. I think my example above is the best way to do that. Have both movies playing at the same time and just hide/show the layers they are on. They should stay in sync with each other (and therefore with the audio file you have running.
If that doesn't work for you, it might be possible to use Ren'py's
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system to queue up the audio files so when you switch movies with the imagebutton the proper audio file starts right away. But that means the files will start over when you switch.
AFAIK, Ren'py has no way to create 'key frames" in movies to sync up audio or other actions.
There's also a
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function in Ren'py that may help you. Honestly though, I have no experience with this particular function yet. From just scanning the docs and some examples, it seems it syncs up separate audio tracks so when you switch between them, they are always in sync with each other. That would be handy if you have different tracks you wanted to use for different movies, but I am not sure it helps you in this example.
If you want to look it over yourself, check
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webpage out. This person has one of the best examples of how to use this function I have seen so far.