c1.css

body {
  background: -webkit-linear-gradient(top, #fff, #000);
  background: -moz-linear-gradient(top, #fff, #000);
  background: -o-linear-gradient(top, #fff, #000);
  background: -ms-linear-gradient(top, #fff, #000);
  background: linear-gradient(to bottom, #fff, #000);
}

c2.css

body {
  background: -webkit-linear-gradient(top, #fff, #000);
  background: -moz-linear-gradient(top, #fff, #000);
  background: -o-linear-gradient(top, #fff, #000);
  background: -ms-linear-gradient(top, #fff, #000);
  background: linear-gradient(to bottom, #fff, #000);
}

Gruntfile.js

module.exports = function(grunt) {

	grunt.loadNpmTasks('grunt-contrib-stylus');

	grunt.initConfig({
		stylus: {
			options: {
				compress: false
			},
			basic: {
				files: {
					"c1.css": "s1.styl"
				}
			},
			dropzone: {
				files: {
					"c2.css": "s2.styl"
				}
			}
		}
	});

	grunt.registerTask('default', ['stylus']);

};

s1.styl

@import 'nib';

body {
  background: linear-gradient(top, white, black);
}

s2.styl

@import 'nib';

body {
  background: linear-gradient(top, white, black);
}