waking_dream made this masked double exposure in two stages: one was made with a round piece of tape blocking the center of the lens, while the other was made holding a card with a circle cut out in front of the lens. Expose the center and then everything except the center. Very cool!
For exposing the middle, you could cut a hole in your lens cap rather than using a card (obviously more appealing if you use a Holga as he did).
Check out his photos, too; the man’s got skills.