<div class="wrapper">
  <div class="item item-1"></div>
  <div class="item item-2"></div>
</div>

body {
  background-color:#ccc;
}

.wrapper {
  background-color: #fff;
  padding: 40px;
  max-width: 800px;
  margin: 40px auto;
}

.item {
  width: 50%;
  height: 300px;
  margin-bottom: 40px;
  background-color: limegreen;
}

@keyframes enter-left {
  from {
    opacity: 0;
    transform: translateX(-50%);
  }
  to {
    opacity: 1;
    transform: translateX(0%);
  }
}

@keyframes enter-right {
  from {
    opacity: 0;
    transform: translateX(50%);
  }
  to {
    opacity: 1;
    transform: translateX(0%);
  }
}

.item-1 {
  animation: enter-left 2s ease-out infinite;
}

.item-2 {
  margin-left: 50%;
  animation: enter-right 2s ease-out infinite;
}