How to apply gulp-clean-css to a less+css stream?

Question

css

In Gulp I did the following task:

gulp.task('css', function() {
    return streamqueue({ objectMode: true },
            gulp.src('src/css/*.css'),
            gulp.src('src/less/*.less')
            .pipe(less())
        )
        .pipe(prefix('last 2 versions'))
        .pipe(concat('production.min.css'))
        .pipe(cleanCSS({ level: 2 }))
        .pipe(gulp.dest('./dist/css'))
        .pipe(reload({ stream: true }))
});

And everything even works fine, but, with this approach, clean-css doesn’t use anything except minification, that is, for example, it doesn’t collect selectors with the same css properties.
How to make clean-css work fine for both less and css?