Create a Nether portal in the Overworld, travel to the Nether. Move at least 16 blocks within the Nether (but not much more), and build a second return portal, then use it, returning to the Overworld. This will create a second portal in the Overworld, some 128-plus blocks from the first. You can then mine the new portal and return to the first one. Returning to the Nether, you will find both portals still functional, and using the second will again create a new portal in the Overworld. This can be repeated as many times as you like.