There were two boys and two girls! My German is largely lost, but here are the essentials. In 1915, Hans Leip and Klaas Deterts met, two young teachers. They were posted to the Gardefüsilieren in Berlin . There they met two young women, Lili and Marleen. Lili was a salesgirl, Marleen was a doctor's daughter who helped out in a hospital. Their association with the girls was brief, the men were sent off to war.
Hans Leip wrote later that, while waiting for a march to resume, he wrote the poem by the light of a streetlamp in April, 1915. He was both lovesick and homesick. He penned "Und sollte mir un Leid gescheh'n, wer wird bei der Laterne steh'n, mit dir, Lili Marleen?" Leip provided a melody for his composition, but it was seldom played.
Later on in life, Leip wrote a novel that was highly praised by Thomas Mann. The two young men had little contact after their brief friendship in 1915.
Note: Both Lili and Lilli and both Klaas and Klaus in the text.
Maybe Wolfgang will add to my skeletal story. Deterts Leip
The original poem may be found in Google, in German websites.