What is the ruling on the prayer of one who prayed 'Isha, combining and shortening it, with a congregation praying Maghrib, where he caught the second rak'ah of their prayer? Is his prayer valid?

The 'Isha' prayer cannot be shortened (qasr) behind someone praying Maghrib, because Maghrib is a complete prayer. It is not permissible to make up the 'Isha' prayer as two rak'ahs while at home; rather, it must be made up as four. It is permissible to recite both tashahhuds in the two rak'ahs when following the imam. It is not permissible to repeat the prayer merely due to doubt.