Edit in JSFiddle

<script src="https://www.youtube.com/iframe_api"></script>
<script>
var movie01
var movie02
function onYouTubeIframeAPIReady() {
	   movie01 = new YT.Player('movie01', {
                height: '315',
                width: '560',
                videoId: 'LBJ0K6J_Xbg',
                events: {
                    'onReady': onPlayerReady,
                    'onStateChange': onPlayerStateChange
                },
                                playerVars: {
                                    "rel":0,            // 関連動画の有無(default:1)
                                }
            });
			movie02 = new YT.Player('movie02', {
                height: '315',
                width: '560',
                videoId: '9-yHfgg91oY',
                events: {
                    'onReady': onPlayerReady,
                    'onStateChange': onPlayerStateChange
                },
                                playerVars: {
                                    "rel":0,            // 関連動画の有無(default:1)
                                }
            });
 
   }

        // 4. The API will call this function when the video player is ready.
        function onPlayerReady(event) {
            event.target.mute();
        }

        // 5. The API calls this function when the player's state changes.
        //    The function indicates that when playing a video (state=1),
        //    the player should play for six seconds and then stop.
        var done = false;
        function onPlayerStateChange(event) {
//            if (event.data == YT.PlayerState.PLAYING && !done) {
//                setTimeout(stopVideo, 6000);
//                done = true;
//            }
        }
        function stopVideo() {
            player.stopVideo();
        }
</script>
  <body>
<div id="movie01"></div>
<div id="movie02"></div>
  </body>