An alternative method is to render the mirrored scene as a jpg/gif file and use the image as a texture on one-sided square. Most image editors contain a mirror function. If you need to animate the image, then you can create lots of one-sided squares, each with a frame of the mirrored action. place the appropriate frame of the mirrored scene in a visible position and hide the other frames behind it. The only problem with this method is it seems to hit a limit at about 370 images (on my machine), so you have to chunk your animations at this limit instead of the usual 999 limit.