<div class="div1">دایو اول<div class="div2">دابو دوم</div></div>
.div1, .div2{width: 150px;height: 150px;background-color: gray;}
.div2{background-color: black;display: none;}
.div1:hover .div2{display: block;}
<div id="div1" style="width: 80px; height: 20px; background-color: red;" onmouseover="document.getElementById('div2').style.display = 'block';document.getElementById('div1').style.display = 'none'">
</div><div id="div2" style="width: 80px; height: 20px; background-color: blue;display: none;">Text</div>