Marlene, Helene, etc. in German have three syllables, ending in "lay-na." Marleen has two syllables. "Marleen" in the German original rhymes better because the line has the right count.
Hans Leip was a poet and author, as pointed out earlier. The two girls may have been imaginary.
I would like to see the original poem (in German). So far, I haven't been able to find it.
Wolfgang? Bitte, por favor! If you have it.