Is it possible to create music by adding 2 or more YouTube videos together?

 At your risk.

For example playing Vocalise - Rachmaninoff (orchestra version) at some interval of time together with Nothing Else Matters - Metalica, both written in E minor.

For example hitting play on Nothing Else Matters when Vocalise is at 5:49 and leaving both videos to play together and interact.

Like an online music mush up game.

Playing music with YouTube videos.

Using YouTube as a musical instrument...

For example having 7 videos each with one of the seven notes sung by a choir. And playing notes just by hitting play on a video while say other 2 are already playing. Or creating clusters like the song I'm Not in Love - 10CC, where they played the mixing desk as a musical instrument, each fader adjusting one recorded multiple tracked vocal note [1]:

Or if YouTube video would allow vari-speed for change of pitch, or change of tempo of a drum loop, or quick repeat of selected intervals of a video, so that the video sound can be sampled and used as grain for Granular Synthesis as I think is described by Yiannis Xenakis.

Or looping say 3 videos each with different duration for creating something that repeats after very long time, as described by Brian Eno.

Or if directly recording eg a vocal or sampling on YouTube.

Or if 2 remote users interact by jamming.

Would some part of future music involve listeners being active participants on music creation. For example given the opportunity to mix online.

But having written this, there is nothing like humans interacting or playing music in the same room, beach, square, forest, space etc.

Dreams - Fleetwood Mac 

Reference:  Good vibrations - Mark Cunningham