<div class="item"></div>

.item {
  width: 50px;
  height: 50px;
  background: linear-gradient(-45deg, red 50%, transparent 50%);
  position: relative;
}

.item:after {
  content: "";
  width: inherit;
  height: inherit;
  position: absolute;
  left: 100%;
  top: 0;
  background: linear-gradient(45deg, red 50%, transparent 50%);
}