.container {
    display: grid;
    background-color: green;
    width: 100%;
    height: 400px;
    justify-content: center;
    align-items: center;
}

.div1,
.div2 {
    width: 200px;
    height: 200px;
    background-color:gray;
    margin: 10px;
    opacity: 0;
    transition: opacity 2s ease-in-out;
    grid-row: 1;
    grid-column: 1;
}

.div1.visible,
.div2.visible {
  opacity: 1;
  transition: opacity 2s ease-in-out;
}