見到木星上面有兩個木衛影

[David]

請問此現象常有嗎?

Here are some dates:
http://www.koshko.com/almanako/jovian_satellites.shtml

Note that there are lots of double shadow transits in Jan 2009. There is an approximately 13 month gap between periods with lots of double shadow transits.

While this event is not really rare, the chance of observing it is small because it may occur during unfavourable times (day time or early morning just before sunrise) and weather may not cooperate. If photography is your goal, you even want this to happen exactly when Jupiter is as high in the sky as possible.