The German submarine fired only one torpedo, not two. The second explosion originated from within the ship near where the torpedo hit. The cause still remains uncertain. It has been widely speculated that the second explosion was caused by war munitions the ship was illegally transporting for the Western Front.
It was German submarine U-20 that torpedoed RMS Lusitania, not German submarine U-39 as shown on the inter-title card. However, on the same day (7 May 1915) U-39 shelled and sunk the United Kingdom trawler Benington in the North Sea. Her crew survived.