get last part of selector and Then use the double ampersand mixin.

Last-part-selector

/// 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;
  }
}