JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrapper">
    <div class="left-block"></div>
    <div class="line"></div>
</div>
<div class="wrapper">
    <div class="right-block"></div>
    <div class="line"></div>
</div>

CSS

.wrapper {
    position: relative;
    overflow: hidden;
}
.left-block, .right-block {
    width: 50px;
    height: 100px;
}
.left-block {
    background-color: red;
    float: left;
}
.left-block + .line {
    left: 50px;
}
.right-block {
    background-color: green;
    float: right;
}
.right-block + .line {
    right: 50px;
}
.line {
    border: 1px solid black;
    box-sizing: border-box;
    content:"";
    margin-top: -1px;
    position: absolute;
    top: 50%;
    width: 100%;
}