<===> input.scss
%foo,
.foo {
 display:block;

  &--up {
   border: none;
 }
}

.zoo {
  @extend %foo;

  &--up {
    @extend %foo--up;
  }
}
<===> output.css
.zoo,
.foo {
  display: block;
}
.zoo--up,
.foo--up {
  border: none;
}