Can you stack teleconvertors? ... yes, and in a pinch they work. ....
canon EF 70-200 f/2.8L IS + canon 1.4 II + canon 2x II tc for560mm (896 in 35mm equiv with the 20d 1.6 crop factor),
the camera doesn't see the 1.4 so the exif shows 400mm and f/8 when it's really f/11