I have installed eac3to by moving it to "c ...program files (x86) " and can use it, but can't seem to figure out how to install this GUI

This is what I will do with the GUI -

1) Take a bunch of files (.mkv, .avi etc; more the better)

2) Convert their 5.1 DTS or 5.1 AC-3 tracks to stereo AAC or MP3 tracks ...

3) ... while keeping the original audio too (NOT default; secondary track; track no.2)

4) Example -
There are 10 similar .mkv files with 5.1 DTS audio tracks and whatever video, subtitles etc (doesn't matter)
Now I use the GUI such that, that after conversion, every file has 2 tracks-
1) Track 1 (or technically track 0 I guess) - is the default stereo MP3 or AAC track
2) Track 2 (or technically track 1 I guess) - is the original secondary untouched 5.1 DTS audio track

If I'm not wrong, this would require associating this GUI with mkvtoolnix. I would need help with that too

People have done it so that they select multiple files, click a button and files are encoded according to the prescribed settings.
I want to do the same.

Any help would be appreciated