SOURCE

<div class="container">
    <div class="row">
        <div class="col">
            <div class="box1">xxxx</div>
        </div>
        <div class="col">
             <div class="box2">yyyy</div>
        </div>
    </div>
</div>

body{
    margin: 0;
    padding: 0;
}

.container{
    /* padding: 0 15px; */
}

/*
   margin-left:负值，自身向左移动
    margin-right:负值，右边的元素向左移动
    注意：这里不加width:100%
 */
.row{
    display: flex;
    /* margin-left: -15px;
    margin-right: -15px; */
    
}

.col{
    width: 50%;
    padding: 0 15px;
}

.box1{
    border-bottom: 1px solid red;
}

.box2{
    border-bottom: 1px solid blue;
}