<div class="center">
<div class="cool_effect">
   <img src="https://images.pexels.com/photos/4124367/pexels-photo-4124367.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=650&w=940" width="400" height="600">
  <img src="https://images.pexels.com/photos/6499190/pexels-photo-6499190.jpeg?auto=compress&cs=tinysrgb&dpr=2&h=650&w=940" width="400" height="600">
</div></div>

body {
  background-color: black;
}

.center {
  text-align:center;
  display:block; 
}

.cool_effect img:first-child {
  position: absolute;
  mix-blend-mode: soft-light; 
}