Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.12)
// Compass (v1.0.3)
// ----

/// Grab the last part of a selector
///
/// @param {List} $selector
/// @returns String
@function x-tail($selector) {
  @return nth(nth($selector, -1), -1);
}

/// Double
/// @param {Number} $margin [null]
@mixin x-doubly($margin: null) {
  & + #{x-tail(&)} {
    margin-left: $margin;
    @content;
  }
}

// Simple usage
ul li {
  @include x-doubly(10px) {
    foo: bar;
  }
}

// Advanced usage
@mixin my-list-item($selector) {
  #{$selector} {
    // This won't be helpful, it outputs .list .item + .list .item
    // And won't style two consecutive children items
    & + & {
      color: hotpink;
    }

    // We actually want to style two consecutive children
    // .list .item + .item
    @include x-doubly {
      color: hotpink;
    }
  }
}

@include my-list-item($selector: '.list > .item');

// (Kudos to @HugoGiraudel for the refactor)

SassMeister-output.css

ul li + li {
  margin-left: 10px;
  foo: bar;
}

.list > .item + .list > .item {
  color: hotpink;
}
.list > .item + .item {
  color: hotpink;
}