Honestly, I don’t know why this might be needed, but since such a service exists, you need to tell about it.
So, if you need to play two, three simultaneously … eight YouTube videos with audio synchronization and other goodies, then there is a YouTube Multiplier service for this.
After going to the site, you need to provide links to all videos, specify the start time and volume. And also choose one of six layouts for displaying video.
Additionally, you can specify a name for the mix, add your name, specify whether you require precise-flow synchronization of the video. And fill in the captcha, where without it.
All mixes become public and available via links like this.