I have two different videos. Now here's the thing, they have the same lengths, and they are related to each other. Is there a way that I can play these videos on at the same time using VLC and also have a unified seek bar, so that the times are the same for both, since one video is dependent on the other.
If anyone is still interested in this the current version of VLC is able to do this very well without any kind of scripting or add-on. To accomplish this simply open VLC and select Media>Open Multiple Files. Inside the multiple file windows select add to add the first file. After this click on the, "Show more options," checkbox followed by the, "Play another media synchronously," checkbox at the bottom of the window. Select browse next to, "Extra media," to add the second file. Hit play and the two media files will play simultaneously with a single control window.
I haven't seen any player that could do that, but you could merge the two video files together to achieve what you want. See questions like https://stackoverflow.com/questions/9293265/ffmpeg-2-videos-transcoded-and-side-by-side-in-1-frame
I have recently solved it with mplayer and you can see the solution on superuser here: master-slave with mplayer
You can use AutoHotKey to achieve something similar though not perfect. I posted my answer to a question similar to yours here
You can do this in VLC for mac (version 2.2.6 https://get.videolan.org/vlc/2.2.6/macosx/) using File > Advanced open file.