Edit in JSFiddle

<div class="a"></div>
<div class="b"></div>
<div class="BFC">
  <div class="a"></div>
</div>
<div class="BFC">
  <div class="b"></div>
</div>

.a {
  width: 100px;
  height: 100px;
  margin: 50px;
  background-color: #ddd;
}
.b {
  width: 100px;
  height: 100px;
  margin: 50px;
  background-color: #eee;
}
.BFC {
  overflow: hidden;
}