JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="center-piece">
    #1
  </div>
  <div class="center-piece2">
    #2
  </div>
</div>

CSS

.container {
  height: 1200px;
  background-color: rgba(0, 0, 0, .7);
  color: white;
  display: flex;
  justify-content: center;
  align-items: center;
}
.center-piece {
  background-color: green;
  position: sticky;
  top: 10px;
  bottom: 50%;
  left: 50%;
  transform: translate(-50%,50%);
}
.center-piece2 {
  background-color: steelblue;
  position: sticky;
  bottom: 10px;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}