How do you open multiple videos with multiple subtitles in VLC from command-line? Usually, you can use this command to open a video with a subtitle in VLC, if it isn't detected automatically:
vlc.exe path_to_video.mp4 --sub-file path_to_sub.srt
But I want to open multiple videos, each with their own subtitle file. How do I do that?
Neither vlc.exe video1.mp4 video2.mp4 --sub-file sub1.srt sub2.srt
nor vlc.exe video1.mp4 --sub-file sub1.srt video2.mp4 --sub-file sub2.srt seem to work.
This is a workaround which converts the files to another container format. It happens almost as fast as your hard disk can write data because nothing is re-encoded.

use MKVToolNix (specifically mkvmerge) to combine the mp4 files and their srt files into mkv files (perhaps you can automate this with a loop). I never use this tool via command line and cannot tell you the exact command, but you'll find more information here https://mkvtoolnix.download/doc/mkvmerge.html
use vlc.exe *.mkv afterwards.

Maybe this question helps too https://superuser.com/questions/609113/how-to-add-and-remove-subtitles-in-an-mkv-file which should also work with an mp4 file as input.