If you used Anaglyph glasses to block red and blue, then you could make your tetris game where falling blocks were red and the landed blocks were blue to achiev

If you used Anaglyph glasses to block red and blue, then you could make your tetris game where falling blocks were red and the landed blocks were blue to achieve the effect they talked about. It'd take some tweaking to get it perfect. If you wanted to make the cardboard idea work, you could make a tetris with two displays, where one side shows falling blocks and the other side shows that they've landed. This seems like it'd be harder to pull off correctly than the other idea.