When importing a series of png images with transparent background as a slideshow clip, the background renders as black... not displaying the underlying clip.
Importing a single png image with transparency as a clip keeps the transparency information if you tick the box "transparent background"... however there is no such box in 0.7.5 for slideshow clips.
BlueScreen effect is not useable as a workaround (i.e. filtering black color does not make the background transparent), maybe because the real background of the images is transparent and not black.
Transparent effect seems to have no effect either.
Details are available at http://kdenlive.org/mantis/view.php?id=1156, but I'd like to ask for **any workaround to composite 2 full-screen videos in a transparent-background-like way**.
PS: wanna try? feel free to use my snow-falling pics attached (hard to see on white... I know), made with synfigstudio (http://synfig.org) particles :)