all.css

@import "one.less";
@import "two.less";

gruntfile.js

module.exports = function(grunt) {
    grunt.loadNpmTasks('grunt-robocopy');
 	grunt.loadNpmTasks('grunt-contrib-less');
	grunt.loadNpmTasks('grunt-contrib-watch');
	grunt.loadNpmTasks('grunt-contrib-clean');
 
	grunt.registerTask('default', ['watch']);
	grunt.registerTask('build', ['clean', 'robocopy']);
	
	grunt.initConfig({
		less: {
			development: {
				options: {
					compress: true,
					yuicompress: true,
					optimization: 2
				},
				files: {
					"src/all.css": "src/all.less"
				}
			}
		},
		clean: ['build'],
		robocopy: {
			options: { 
				source: 'src',
				destination: 'build',
				files: ['*.html', '*.png', '*.jpg', '*.gif', '*.css', '*.ico'],
				copy: {
					mirror: true
				},
				retry: {
					count: 2,
					wait: 3
				},
			}
		},
		watch: {
			styles: {
				files: ['src/*.less'],
				tasks: ['less'],
				options: {
					nospawn: true
				}
			}
		}
	});
};

one.less

one {
	background: green;
}

two.less

two {
	background: blue;
}