gruntfile-snippet.js

mytask: {
  options: {},
  a: {
    options: {
      src: "src",
      dest: "dest",
      match: '*'
    }
  },
  b: {
    options: {
      src: "src2",
      dest: "dest2",
      match: '*'
    }
  }
},
oldtask: {
  options: {
      src: '',
      dest: ''
  }
}

grunt.task.registerTask('mytask', 'mytask descr', function () {
  // get options here
  this.options() // -> won't work with grunt mytask:a, but did work with grunt oldtask
})