grunt-runner

gistfile1.js

var runner = require('grunt-runner');
var concat = require('grunt-contrib-concat');
var config = {
  src: 'path/to/src',
  concat: {
    targetOne: {
      src: '<%= src %>/one/*.js',
      dest: 'output1.js'    
    },
    targetTwo: {
      src: '<%= src %>/two/*.js',
      dest: 'output2.js'
    }
  }
};
var concatTask = runner(concat, config);

concatTask('concat'); // yields two functions
concatTask('concat:targetTwo'); // yields one function
concatTask('concat --verbose'); // yields two verbose logging functions

var adapter = require('grunt-adapter');
var tasks = adapter.loadNpmTasks('grunt-contrib-concat');
var config = {
  src: 'path/to/src',
  concat: {
    targetOne: {
      src: '<%= src %>/one/*.js',
      dest: 'output1.js'    
    },
    targetTwo: {
      src: '<%= src %>/two/*.js',
      dest: 'output2.js'
    }
  }
};
var concatTask = runner(tasks.concat, config);

concatTask('concat'); // yields two functions
concatTask('concat:targetTwo'); // yields one function
concatTask('concat --verbose'); // yields two verbose logging functions