Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="container">

  <div class="A"><img src="http://placehold.it/1000x500" /></div>
  
  <div class="B">
    <div class="C"><img src="http://placehold.it/500x500" /></div>
    <div class="D"><img src="http://placehold.it/500x500" /></div>
  </div>

</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@import "susy";

.container {
  width: 1000px;
  margin: 0 auto;
}

.A,
.B {
  @include span(12 of 12);
  padding: 0 20px;
}

.A {
  margin-bottom: gutter(12);
}

.C,
.D {
  @include span(6 of 12);
}

.D {
  @include last;
}

SassMeister-output.css

.container {
  width: 1000px;
  margin: 0 auto;
}

.A,
.B {
  width: 100%;
  float: left;
  margin-left: 0;
  margin-right: 0;
  padding: 0 20px;
}

.A {
  margin-bottom: 1.69492%;
}

.C,
.D {
  width: 49.15254%;
  float: left;
  margin-right: 1.69492%;
}

.D {
  float: right;
  margin-right: 0;
}

SassMeister-rendered.html

<div class="container">

  <div class="A"><img src="http://placehold.it/1000x500" /></div>
  
  <div class="B">
    <div class="C"><img src="http://placehold.it/500x500" /></div>
    <div class="D"><img src="http://placehold.it/500x500" /></div>
  </div>

</div>