If I understood your problem correctly, then below code will work for you.
Style:
<style type="text/css" media="screen">
#midDiv
{
position: relative;
border: 1px solid #ccc;
width: 500px;
height: 250px;
top: 100px;
left: 200px;
}
#div1
{
top:10%;
left:0%;
background-color:
red;
width:20%;
bottom:0%;
position: relative;
}
#div2
{
top:10%;
left:20%;
background-color:
blue;
width:80%;
bottom:0%;
position: absolute;
}
HTML:
<div id="midDiv">
<div id="div1">Div1</div>
<div id="div2">Div2</div>
</div>
Also I suggest not to use inline style as it is bad coding practice.
Happy Coding!