<html>
<head>
<style>
:root {
  --one: calc(var(--two) + 20px);
  --two: calc(var(--one) - 20px);
}}
</style>
<body>
<script>
document.write("One: " + getComputedStyle(document.documentElement).getPropertyValue("--one") + "Two: " + getComputedStyle(document.documentElement).getPropertyValue("--two"))
</script>
</body>
</html>