You can download it by clicking Here!
Run AviMuxGui and you'll see this screen:

Now Rightclick in the topbox and after that click on add.
Choose your avi's as shown in the next screens:




Select the 2 avi files in the top box of AviMuxGui by clicking on them while holding the Ctrl. The files will turn on blue after selecting them.

After that click on: "generate data source from files"
Your screen will look like this:

Click on start, choose a name for the joined avi (it most be a different name as the name for the single 2 avi's) and choose save as.

And now AviMuxGui is joining the files. When it is finished you've got a perfect joined file. It will leave no mark at all on the new file.