Overlay Divs Without Absolute Position

.parent {
  display: grid;
  grid-template-columns: 1fr;
}

.parent div {
 padding: 50px;
 background-color: rgba(0,0,0,0.5);
 grid-row-start: 1;
 grid-column-start: 1;
}

<div class="parent">
  <div class="child1">
  1
  </div>
  <div class="child2">
  2
  </div>
</div>