Mimic?
Thanks for the list of special names!
Well here is my original idea in more mathematical details.
Basically I wanted 2 rooms side-by-side, with a sheet of glass on the Y axis, which would act as the axis of symmetry. So if the player places a blue portal at coordinates (5,2,1), a blue portal of ID 1 would appear at coordinates (-5, 2, 1). It would be the same for all entities, including the player, picked up boxes, etc. Puzzles would have revolved around the differences between the 2 rooms, and then would gradually become more complex.
Doing such a thing is probably impossible, but I was wondering how close I could get to this original idea.
That sounds pretty complicated, especially keeping all the props synchronised between both halves of the room. And I have no idea how it could be possible to make the alternate portals match up with the original ones.
Just out of curiosity: what would happen when the player carries a box into an area that is solid in the mirror image? (see drawing) Would it cause an anomaly in the fabric of space which would lead to the entire universe being sucked through a dime-sized hole?
(As per your description, the two rooms are NOT mirror images of each other 
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
Very good point, I already considered the problem.
To avoid undesired desync issues, I was thinking of having collision and portal surfaces identical on both sides. To achieve that, there would be several invisible objects.
Well, you could always keep both rooms the same and only change 'non-colliding' things, like the indicator lines.
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic
http://www.crazymonkeygames.com/Visible.html
Like that, right?
