Combine multiple AVIs:
MENCODER:
mencoder -oac copy -ovc copy -o output.avi input1.avi input2.avi
FFmpeg:
From the FFmpeg Wiki article on how to concatenate (join, merge) media files:
Create a file "mylist.txt" with all the files you want to have concatenated in the following form ( Lines starting with a dash are ignored ) :
# this is a comment file '/path/to/file1' file '/path/to/file2' file '/path/to/file3'
Note that these can be either relative or absolute paths. Then you can encode your files with:
ffmpeg -f concat -i mylist.txt -c copy output