SassMeister-input-HTML.html

<div class="block">
  <div class="block__element">Stuff</div>
  <div class="block__element--modifier">Red Stuff</div>
  <div class="block__element--and-another">Yellow Stuff</div>
  <div class="block__element--modifier block__element--and-another">Blue Stuff</div>
</div>

SassMeister-input.scss

// ----
// Sass (v3.3.9)
// Compass (v1.0.0.alpha.20)
// ----

.block {
  &__element {
    background-color: #000;
    color: #fff;
    
    &--modifier {
      @extend .block__element;
      color: #f00;
    }
    &--and-another {
      @extend .block__element;
      color: #ff0;
    }
    &[class*="--modifier"][class*="--and-another"] {
      color: #00f;
    }
  }
}

SassMeister-output.css

.block__element, .block__element--modifier, .block__element--and-another {
  background-color: #000;
  color: #fff;
}
.block__element--modifier {
  color: #f00;
}
.block__element--and-another {
  color: #ff0;
}
.block__element[class*="--modifier"][class*="--and-another"], [class*="--modifier"][class*="--and-another"].block__element--modifier, [class*="--modifier"][class*="--and-another"].block__element--and-another {
  color: #00f;
}

SassMeister-rendered.html

<div class="block">
  <div class="block__element">Stuff</div>
  <div class="block__element--modifier">Red Stuff</div>
  <div class="block__element--and-another">Yellow Stuff</div>
  <div class="block__element--modifier block__element--and-another">Blue Stuff</div>
</div>