Refer below code and use it. it is 100% working........
<style>
.div_1
{
float: left;
min-width: 350px;
width: 50%;
}
.div_2
{
float: right;
min-width: 350px;
width: 50%;
}
@media screen and (max-width: 800px) {
.div_2
{
float: none;
min-width: 350px;
width: 100%;
}
}
</style>
<div class="div_2">div 2</div>
<div class="div_1">div 1</div>