JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a" class="inversePair">A</div>
<div id="b" class="inversePair">B</div>

CSS

body {
    padding: 10px;
}
.inversePair {
    border: 1px solid black;
    background: grey;
    display: inline-block;    
    position: relative;    
    height: 100px;
    text-align: center;
    line-height: 100px;
    vertical-align: middle;
}

#a {
    width: 100px;
    border-radius: 50px;
    z-index: 1;
}

#a:before {
    content:' ';
    left: -6px;
    top: -6px;
    position: absolute;
    z-index: -1;
    width: 112px; /* 5px gap */
    height: 112px;
    border-radius: 56px;
    background-color: white;
} 

#b {
    width: 200px;
    z-index: 0;
    padding-left: 50px;
    margin-left: -55px;
    overflow: hidden;
}

#b:before {
    content:' ';
    left: -58px;
    top: -7px;
    position: absolute;
    z-index: 1;
    width: 114px; /* 5px gap, 1px border */
    height: 114px;
    border-radius: 57px;
    background-color: black;
}