Well, i can only do that when its a separate line. When i use the dot to make it a property of the clip like 'clip.overlay(....)' i think it already takes clip as its first parameter, at least, i cant get it to work with two clips.
Anyway, it seems as if the overlay function is a little different then rendering it directly on the video with AE. I got two different results:
Kinda weird, it appears a little different too.
When overlaying karaoke, you get the same problem when you let lines fadeout. Rendering it directly onto the video with AE gives you the results you expect, but using overlay makes it darker when it fades out, almost black. It feels different, less smooth
I guess AE uses a different algorithm or there are some problems with colorspaces when using avisynth, but it's definately not the same (here).
EDIT:
Could it possibly be something with premultiplied vs straight?