#firstDropContainer{
float: left;
width: 40%;
margin-right: 1.5em;
}
#secondDropContainer{
float: left;
width: 40%;
margin-bottom: 1em;
}
<div id="mainDrop">
<div id="firstDropContainer"></div>
<div id="secondDropContainer"></div>
</div>
Note: Adjust the width of the divs based on your req.