<div class="div-container">
  <div class="div1">1</div>
  <div class="div2">2</div>
</div>

/* 方法一 */
.div-container {
  margin: 10px 0;
  padding: 10px;
  width: 400px;
  border: 2px solid #ccc;
  position: relative;
}
.div1 {
  width: 100px;
  height: 50px;
  border: 2px solid red;
}
.div2 {
  width: 100px;
  height: 50px;
  border: 2px solid red;
  position: absolute;
  /* 边距设置 */
  right: 10px;
  top: 10px;
}

<div class="div-container2">
  <div class="div3">3</div>
  <div class="div4">4</div>
</div>

/* 方法二 */
.div-container2 {
  margin: 10px 0;
  padding: 10px;
  width: 400px;
  border: 2px solid #ccc;
  display: flex;
  justify-content: space-between;
}
.div3 {
  width: 100px;
  height: 50px;
  border: 2px solid red;
}
.div4 {
  width: 100px;
  height: 50px;
  border: 2px solid red;
}

<div class="div-container3">
  <div class="div5">5</div>
  <div class="div7">占位div</div>
  <div class="div6">6</div>
</div>

/* 方法三 */
.div-container3 {
  margin: 10px 0;
  padding: 10px;
  width: 400px;
  border: 2px solid #ccc;
  display: flex;
  justify-content: space-between;
}
.div5 {
  width: 100px;
  height: 50px;
  border: 2px solid red;
}
.div6 {
  width: 100px;
  height: 50px;
  border: 2px solid red;
}
.div7 {
  width: calc(100% - 100px - 100px);
  height: 50px;
  border: 1px solid #ccc;
}