9.5 “Draco dormiens nunquam titillandus”

Autoprefixer 9.5 brings mask-composite support.

a {
  mask-composite: add;
}
b {
  mask: url(intersect.png) intersect, url(exclude.png);
}

a {
  -webkit-mask-composite: source-over;
          mask-composite: add;
}
b {
  -webkit-mask: url(intersect.png), url(exclude.png);
  -webkit-mask-composite: source-in, xor;
          mask: url(intersect.png) intersect, url(exclude.png);
}

Thanks to @semeleven for implementation and @iamvdo for suggestion.