如何将第二个模态放在第一个模态之上？

<!-- 第一个模态框 -->
<div id="modal1" class="modal">
  <div class="modal-content">
    <span class="close" onclick="openModal2()">&times;</span>
    <h3>第一个模态框</h3>
    <!-- 模态框内容 -->
  </div>
</div>

<!-- 第二个模态框 -->
<div id="modal2" class="modal">
  <div class="modal-content">
    <span class="close" onclick="closeModal2()">&times;</span>
    <h3>第二个模态框</h3>
    <!-- 模态框内容 -->
  </div>
</div>

.modal {
  display: none;
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.4);
}

.modal-content {
  background-color: #fefefe;
  margin: 15% auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}

.close {
  color: #aaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
  cursor: pointer;
}

.close:hover,
.close:focus {
  color: black;
  text-decoration: none;
  cursor: pointer;
}

function openModal2() {
  var modal2 = document.getElementById("modal2");
  modal2.style.display = "block";
}

function closeModal2() {
  var modal2 = document.getElementById("modal2");
  modal2.style.display = "none";
}