SOURCE

<div class="line-box">
       <!-- 横线，测试用，用来比对两个子元素对齐的位置 -->
      <div class="line"></div>
      <div class="child-box1"></div>
      <div class="child-box2"></div>
</div>

.line-box {
    position: relative;
    padding-left: 100px;
    border: 1px solid black;
}
.line-box::after {
    content: 'x';
}
.child-box1, .child-box2 {
    display: inline-block;
    width: 100px;
    height: 100px;
    border: 1px solid red;
}
.line {
    position: absolute;
    top: 100px;
    height: 1px;
    width: 300px;
    background: black;
}