Split rule by selectors

_split.scss

@mixin split($selectors...) {
    @each $selector in $selectors {
        #{$selector} {
            @content;
        }
    }
}

demo.css

p {
  background: #00F;
}
p::selection {
  background: #F00;
  color: #FFF;
}
p::-moz-selection {
  background: #F00;
  color: #FFF;
}
p::-webkit-selection {
  background: #F00;
  color: #FFF;
}

demo.scss

p {
    background: #00F;
    
    @include split('&::selection','&::-moz-selection','&::-webkit-selection') {
        background: #F00;
        color: #FFF;
    }
}