Dual-licensing is when you release your project under 2 (or more) licenses at the same time.
Note that any code you want to integrate in your project, and that you do not own the copyright of, must be compatible with the 2 licenses at the same time.
You also need to be the sole copyright holder of the code if you want to pass from a single-licensing to a dual-licensing scheme.
Example copyright notice, for GNU GPL + EPL:
You may copy, modify, and redistribute this file under the terms of the GNU General Public License version 2 (or, at your option, any later version), and/or the Eclipse Public License version 1.0. [standard GPL notice] [standard EPL notice]