You are correct, this song is about two young lovers. I'd venture that it's a romance built largely around the wordplay in the lyrics.
"Helplessly hoping..." this is a series of alliterations built first on "H" and then on "W," and "G". In the Italian Commedia dell'arte theater tradition, "Harlequin" was the name given to a clown/mime figure who was also usually the young male romantic lead. This stanza is a portrait of a confused (and like the original "Harlequin", speechless) young man who is attracted (as so many of us are) to an elusive lover that is leaving ("sound of goodbye").
"Wordlessly waiting..." this is alliterations again, this time with the W first, and then the H. Now that his lover is gone, he pines and misses her. But has she really left? He doesn't know if his love interest is entering or leaving his life ("hello" or "goodbye").
"Stand by the stairway..." is new set of alliterations, this time "S", "C" and "L". This is the love interest, the Columbine to the Harlequin, an equally confused young lady who is trying to leave, but instead "lingers" feeling "lost." She is drawn to a love connection, but she is "choking" on admitting it.
"They are 1 person..." There's a different word game in the chorus. Each number after the first one is a pun. You should read this as:
They are one person
They are too alone
They are free together
They are for each other
As with many songs, the chorus answers the questions and resolves the confusions of the verses. The lovers may not know if they are destined for each other, but the chorus confirms their pre-ordained love. They are "one person" --that's possibly an allusion to (pseudo) Aristophanes's myth that soulmates are two halves of the same person, from Plato's Symposium. They are lonely, they bring each other freedom, and finally, they are "for each other."