<!doctype html>
<html>
    <head>
        <meta charset="utf-8">
        <style>
            body {
                margin: 0px;
            }

            #outer {
                position: absolute;
                width: 600px;
                height: 200px;
                background-color: aquamarine;
            }

            #left, #right {
                position: absolute;
                width: 300px;
                height: 200px;
                background-image: radial-gradient(closest-side, red 40%, transparent 40%)

            }
            #left {
                left: 80px;
            }

            #right {
                left: 380px;
            }
        </style>
    </head>
    <body>
        <div id="outer">
            <div id="left"></div>
            <div id="right"></div>
        </div>
    </body>
</html>