This is an idea for people who have two rooms of similar size next to each other, don't really need one, and can afford to spend huge amounts of money on ridiculous half-baked ideas.
First, make a large, rectangular hole between the two rooms. It helps if the wall is very thin. Now put a pane of glass
into the gap, exactly between the two rooms.
Pick which room you want to use. Put in cheap (recommended) furniture, and buy two of everything. It helps if you do not try to be artistic and modern, with everything in random places. Now do exactly the same for the other room, but as though the wall is a mirror. For example, if the piano (not advisable) is exactly in the corner to the far corner next to the adjoining wall, put the other piano in the far corner next to the adjoining wall in the other room.
Link up the lights, so that if one light goes on, so does the other. Preferably have an on/off button, rather than a switch.
Hide the other door from the outside, leaving it looking normal from the inside (if you really want, put the door on a motor so that when one door opens, the other does too).
Bring your idendical twin along, and have him practice to copy your movements exactly.
Now for the fun part: Take a friend into your room. As they comment on the lovely new mirror you have, they will slowly percieve something wrong. Suddenly, it will dawn on them that they have no reflection. They will panic. Mwahahahaha!
If you want, you can also...
...get other people to come into the other room, and pretend to be souls trapped inside the mirror.
...have a conversation with your twin.
...put the lights in the other room on a remote dimmer, so that you can turn the lights of there and make the glass act like a normal mirror.
Sorry for the length of this, but I wanted to include everything.