Your files have to be encoded in the same manner to do a simple join in AviDemux…same codec, framerate, resolution, etc. Look in the main window and make sure the Video and Audio settings are set to Copy.
Open the first file by going to File, then hit Open.
Go to File again, hit Append, then import the second file.
Now Save the file and when you make a new name for the file, use the same extension on the file name as the original files…in other words if you are starting with an .avi file make the combined file something like this: “name of new file”.avi