I do Youtube Videos and had this same problem. The way I got around it was this: I record using OBS64 and I put the desktop audio on tracks 1 and 2 and my voice on track 3. I save it as an MP4 file.
I then copy the saved file and save it so I have the same file with 2 different names.
I import the first file into Video Pad Pro while telling it to use Track 1 in stereo (which actually gives me tracks 1 and 2.
Then I import the 2nd file (same file, different name) and I tell that one to use track 3.
I now have 2 parallel videos with separate audio tracks in my lower scroll section. I can "mute" the video (not audio) portion of the 2nd file and that essentially gives me 1 video file with 2 audio tracks: one stereo and one mono for my voice.