JSFiddle - React, Tailwind, and code Playground

HTML

<div class="left"></div>
<div class="right"></div>

CSS

.left, .right{
        float:left;
    }

    .left{
        background: red;
        width: 200px;
        height: 300px;
        transition: width 1s ;
    }

    .right{
        background: blue;
        width: 300px;
        height: 300px;
        transition: width 1s;
    }

    .left:hover{
        width: 300px;
    }

    .left:hover + .right{
        width: 100px;
    }
</style>