[css] Two Divs next to each other, that then stack with responsive change

With a mediaquery based on a min-width you could achieve something like http://jsbin.com/aruyiq/1/edit

CSS

.wrapper { 
  border : 2px dotted #ccc; padding: 2px; 
}

.wrapper div {
   width: 100%; 
   min-height: 200px;
   padding: 10px;
   -webkit-box-sizing: border-box;
   -moz-box-sizing: border-box;
   box-sizing: border-box;
}
#one { background-color: gray; }
#two { background-color: white; }

@media screen and (min-width: 600px) {
   .wrapper {
      height: auto; overflow: hidden; // clearing 
   }
   #one { width: 200px; float: left; }
   #two { margin-left: 200px; }
}

In my example the breakpoint is 600px but you could adapt it to your needs.