Hello,
There have been two threads over at the Windows forum about this. I'm here to report that the same problem is happening with the Mac OS version of VLC too. It seems that the 'play media syncronously' function has not been working since at least 2019.
I've tested on two different computers with the latest version of VLC and the behaviour is identical on both. One computer runs High Sierra and the other Big Sur.
On Mac OS, here is what happens:
1. Press "Command + Shift + O"
2. Select two media files to play in the next window (I've tried various combinations of mov, m4v and mkv files).
3. Press Play
One of two unintended things happen:
1. Either only one of the videos play,
or
2. Two players show up but the same video is in both.
The posts at the Windows section about the same issue:
https://forum.videolan.org/viewtopic.ph ... ncronously
https://forum.videolan.org/viewtopic.php?f=14&t=148225