Rings of teleportation are very handy things to have around. The surface bounded by one ring is equated with the surface bounded by the other, so if you put something through one ring it will come out through the other. (Like the portals in "Portal", but more portable). They don't exist, of course, but this technicality doesn't prevent us from drawing pictures of them.
Writing code to render these things is an interesting exercise. It's easy to do with a ray tracer - if a ray intersects the disc inside one ring, just continue it to the equivalent point on the other ring.
Once that's working, you can put the rings side-by-side so that light goes around in circles - if you put your eye point in the middle you can see an infinite tunnel.
A trick you can play is to reverse the orientation of one of the rings so that you look through one ring, out of the other to an object, the object will appear to you to be inverted, as in a mirror image.
Another trick is to make the rings different sizes, or shapes. As long as there is a 1:1 function equating points on one surface with points on the other, it works fine.
However, having rings of different sizes or non-circular shapes opens the possibility of putting one ring through the other. What happens then? It seems like the "infinite tunnel" then becomes a real thing rather than just an optical effect, but where does the second ring exist in real space? It seems that the only place it can appear is through the other side of the first ring, but that would mean that every point in space appears in an infinite number of places - this seems like it would have rather drastic consequences.
So it seems more likely that the second ring would be prevented from going through the first somehow (perhaps a ring edge would get in the way).