JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block"> 
   <div class="b1"> 
     <button>
	  Кнопка
	 </button>
   </div>
   <div class="b2"></div>
</div>

CSS

.block{
	width:400px;
	height:300px;
	margin:20px auto;
	border:1px solid ;
}
.b1,.b2{
	float:left;
	box-sizing:border-box;
	width:49%;
	height:100%;
	background:red;
}
.block:hover .b2{
	background:blue;
	margin-left:4px;
}