I assume that they are already swimming on the left, or they will not be next to each other. A simple clear: leftC will do the trick:
#B {
float: left;
width: 100px;
height: 200px;
background: #0f0;
margin: 5px;
}
#C {
float: left;
clear: left;
width: 100px;
height: 200px;
background: #00f;
margin: 5px;
}
I added some fields, paddings and backgrounds.
Example: http://jsfiddle.net/ambiguous/uCBYV/