Try a clear: left on #inner2. Because they are both being set to float it should cause a line return.
#inner1 {_x000D_
float:left; _x000D_
}_x000D_
_x000D_
#inner2{_x000D_
float:left; _x000D_
clear: left;_x000D_
}
_x000D_
<div id="wrapper">_x000D_
<div id="inner1">This is inner div 1</div>_x000D_
<div id="inner2">This is inner div 2</div>_x000D_
</div>
_x000D_