2videos.html

<div>
  <video id="a" src="red.mp4" style="width:300px" autoplay></video>
  <video id="b" src="gron.mp4" style="width:300px"></video>
</div>
<button id="btn">play</button>

<script>
  const v1 = document.querySelector('#a');
  const v2 = document.querySelector('#b');
  document.querySelector('#btn').onclick = () => v1.play();
  v1.onended = () => {
    v2.play();
    v1.currentTime = 0;
  }
  v2.onended = () => {
    v1.play();
    v2.currentTime = 0;
  }
</script>