Complex sentence: He picked up the gun and ran to Linnet's cabin, which was on the starboard side of the boat while Cornelia and Mr. Fanthorp brought Jacqueline to Dr. Bessner and Miss Bowers' cabins, which were on the port side.
Split into simpler sentences: He picked up the gun and ran to Linnet's cabin. It was on the starboard side of the boat while Cornelia and Mr. Fanthorp brought Jacqueline to Dr. Bessner and Miss Bowers' cabins, which were both on the port side.