Based on the information provided, the two details that would be included in the middle of the plot diagram are:
- After Gortsby finds the cake of soap on the bench, he tracks down the young man to return the soap and to loan him money for a hotel room.
- After the young man leaves, Gortsby finds a cake of soap lying on the bench and believes that the young man left it there.
These details relate to the development of the plot following the young man's departure and the significance of the cake of soap in Gortsby's decision-making process.