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.