MKVtoolNix GUI "Files cannot be appended to themselves."

Hello

Ive looked everywhere and can't figure out what I am doing wrong.

I am taking the LOTR extended edition blu rays and combining disc 1 and disc 2 (using the method here).

I open the program and add the first half of "fellowship".

I then click on that file and select "append files" and select the second half of fellowship

I change the output to "fellowship full".

However, when I select "start multiplexing" I get an error stating "Files cannot be appended to themselves".

Am I doing something wrong? Ive tried renaming all the files but I keep getting that error.

Any help would be grand.

Steve