Two shadows of Galilean satellites will be simultaneously transiting Jupiter during the evening of September 7th in North America and the early morning of September 8th in Europe. Io (#1) itself will be in transit along with its shadow and that of Callisto (#4). I calculate that the period in which both shadows are fully on Jupiterís disk will only last two minutes from 00:13:28 UT to 00:15:28 UT. In Chicagoland this will occur around sunset, but Jupiter will be above the horizon. Observers to the east will see the dual shadow transits at nighttime and those to the west in daylight.
Iíve created three graphics to illustrate the events. The first is a timetable. The second shows a view from above Jupiterís north pole. The third is a close-up of the view from Earth. They can be seen by clicking: http://www.curtrenz.com/astronomical.html
Pictures and descriptions of the events would be welcome additions to this thread.