Set their widths to 50%. Example: http://jsfiddle.net/52R8R/
CSS
.b{
width:50%;
float:left;
}
.c{
width:50%;
float:right;
}
HTML
<div class="b">aaaaaaaaaaaaaaaaaaaa</div>
<div class="c">11111</div>
<div class="b">aaaa</div>
<div class="c">11111</div>
EDIT: taking into consideration your comment this should work:
.c
{
float:right;
border-style:solid;
}
.b
{
border-style:solid;
background-color:blue;
overflow:hidden;
}
note: you have tu put your divs with <div class="c">
before b.