.item {
width: 100%
}
.container {
display: flex;
flex-wrap: wrap;
}
.container > div {
flex: 50%; /* or - flex: 0 50% - or - flex-basis: 50% - */
/*demo*/
box-shadow: 0 0 0 1px black;
margin-bottom: 10px;
}
/////////////html
<div class="container">
<div class="item">1</div>
<div class="item">2</div>
<div class="item">3</div>
<div class="item">4</div>
</div>