If I have a movie split into two AVI files, I usually encode them to one CD by putting two AviSource() lines in my avs script (separated by +). But how can I use that with Moviestacker to determine my average video bitrate? It can only load one video file at a time. Will it work if I just load both files separately and average the values I get for video bitrate?

The average bitrate is a simple computing based on the length of the file. You don't even have to load the avi to find it !

Just adjust the length of the movie (set it to the length of the whole two movies) in the "Video Stream" tab and read the Average birate you need.

excellent, thanks!