JavaScriptでページ遷移っぽいものを

transitionだけで行うなら

.normal {
  margin-left: 0px;
  -moz-transition: all 1s ease;
  -webkit-transition: all 1s ease;
}

.moved {
  margin-left: -300px;
  -moz-transition: all 1s ease;
  -webkit-transition: all 1s ease;
}

として

<div style="width:300px; overflow:hidden;">
	<div id="w" style="width:600px; display:flex; flex-direction:row; " class="normal">
		<div style="width:300px; background-color:#FFEFEF;">ok1</div>
		<div style="width:300px; background-color:#EFFFFF;">ok2</div>
	</div>
</div>

<div onClick="document.getElementById('w').className='moved';">click</div>

<div onClick="document.getElementById('w').className='normal';">back</div>

こんな感じかな