SOURCE

<div class="box">
    <div class="left"></div>
    <div class="right"></div>
</div>

.box{
    display: flex;
    align-items: center;
}
.left {
    background-color: red;
    width: 50%;
}
.right {
    background-color: green;
    width: 50%;
}
.left,.right{
    height: 300px;
}