overlaying multiple videos (like photoshop layers)

[chichikov]

I'd like to overlay multiple videos (ontop of each other in a single window) (like photoshop layers) with a "add blend" or "alpha over" style.
The reason for this I want to add some effects (eg animated logo) etc but dont want to reencode the videos.

I search a lot for this but couldnt find this as a feature.
If it already exists pls let me know.

Thanks.