This should be possible in most browser (probably not on iOS).
So what I would do is:
At the start action of the first scene I'd add the following custom JS
Code:
//note that you need to give the video an id of "myVideo" in Animator
var videoContainer = controller.getElementById('myVideo');
var videoEl = videoContainer.querySelector('video');
videoEl.play();
Then at the start action of the second scene I'd add the following custom JS
Code:
var videoContainer = controller.getElementById('myVideo');
var videoEl = videoContainer.querySelector('video');
videoEl.pause();