The best GUI program to use to combine .avi release upon my google searching was Avidemux. Why? Because its alot like VirtualDub for windows.



How to with Avidemux: open CD1, append CD2 and click save film.


If your ok with using the terminal than the easiest approach is simply to open the terminal, and insert this command: avimerge -i "/directory/CD1.avi" "/directory/CD2.avi" -o "/directory/film.avi" thats quick easy and painless, keep in mind for whatever reason depending on your security you may need to add sudo the front of that command.