JSFiddle - React, Tailwind, and code Playground

HTML

<button>Двухцветная кнопка</button>

CSS

button{
    position: relative;
    padding: 10px 40px;
    background:red;
    border:none;
   
}

button:before{
    content:'';
    position: absolute;
    height:100%;
    width:20px;
    background:grey;
    left:0;
    top:0;
}