JSFiddle - React, Tailwind, and code Playground

HTML

<body>
<div class="div1">
</div>

<div class="div2">
</div>

<div class="div3">Текст
</div>

</body>

CSS

body {margin:0;padding:0;background-color:#cecece;min-height:1000px;}

.div1 {
  width:100%;
  height:70px;
  top: 0;
	position: fixed;
  z-index:1;
  background-color: #fff;
}
.div1.sm {
  height:50px;
}

.div2 {
  width:100%;
  height:70px;
  top: 70px;
	position: fixed;
  z-index:1;
  background-color: #000;
}
.div2.sm {
  height:50px;
  top: 50px;
}

.div3 {margin-top: 140px;
}

JavaScript

$(window).scroll(function() {
				if ($(window).scrollTop() > 70) {
					$(".div1, .div2").addClass("sm");
				} else {
					$(".div1, .div2").removeClass("sm");
				}
			});