How can I play/pause more than one video by mouseover

Based on the answer of Etienne Miret a minimal implementation does not need a specific function at all.

Simply setting onmouseover="this.play()" and onmouseout="this.pause()" should do the trick:

<div style="text-align:center"> 
  <video id="video1" width="420" onmouseover="this.play()" onmouseout="this.pause()">
    <source src="mov_bbb.mp4" type="video/mp4">
    <source src="mov_bbb.ogg" type="video/ogg">
    Your browser does not support HTML5 video.
  </video>
  <video id="video2" width="420" onmouseover="this.play()" onmouseout="this.pause()">
    <source src="mov_bbb.mp4" type="video/mp4">
    <source src="mov_bbb.ogg" type="video/ogg">
    Your browser does not support HTML5 video.
  </video>
</div> 

Use the this keyword:

onmouseover="playPause(this)"

and in your Javascript:

function playPause(video) {
    if (video.paused) {
        video.play();
    } else {
        video.pause();
    }
}

You need to pass a reference to the video you want to play/pause. f.ex:

<div style="text-align:center"> 
  <video id="video1" width="420" onmouseover="playPause('video1')" onmouseout="playPause('video1')">
    <source src="mov_bbb.mp4" type="video/mp4">
    <source src="mov_bbb.ogg" type="video/ogg">
    Your browser does not support HTML5 video.
  </video>
  <video id="video2" width="420" onmouseover="playPause('video2')" onmouseout="playPause('video2')">
    <source src="mov_bbb.mp4" type="video/mp4">
    <source src="mov_bbb.ogg" type="video/ogg">
    Your browser does not support HTML5 video.
  </video>
</div> 

<script> 

function playPause(videoID)
{ 
var myVideo=document.getElementById(videoID); 

if (myVideo.paused) 
  myVideo.play(); 
else 
  myVideo.pause(); 
} 
</script>