Dual-licensing is when you release your project under two (or more) licenses at the same time.

Any code you want to integrate in your project, and for which you do not hold the copyright, must be compatible with both licenses.

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]