When the satellites of Jupiter rotate, their shadows are sometimes casted on Jupiter. While such 'solar eclipse on Jupiter' is commonplace, it is very rare that two shadows were observable at the same time. However, such double-shadow transit on Jupiter (雙影凌木) was exactly what occurred between 8:56pm and 10:02pm in this cloudless night. Both satellites and their shadows were easily observable, with Ganymede followed by Io. Their motion around Jupiter was very noticeable, and we were particularly amazed by the fastest moving Io which crossed the whole disk of Jupiter between 8:00pm and 10:16pm. The steady and exceptionally transparent atmosphere allowed us to take pictures at very high resolution. This was undoubtedly Jupiter observation at its best.