Both works depict challenges with being gay in South Korean society as well as experiences of familial relationships for gay people, and both works do so with a rather literary approach to writing (focusing on arranging words in a beautiful and profound way); a somewhat melancholic but also hopeful ending and overall emotional tone; and an achingly beautiful instrumental score for the soundtrack. The writing similarities are strongest with episodes 3-4 of "Love in the Big City", which (like "So Long, See You Tomorrow") makes heavy use of universe-related imagery.
If you take Love in the Big City's episode-6 relationship arc; combine it with episode 8's nostalgic, melancholic, and temporally disjointed remembrance of times past (and idealized expectations pulled back down to reality) on vacation in a foreign land; intensify the moody sense of loneliness and dislocation; make the humor a bit more subtle; keep the attention to grittier dimensions of gay life, the more explicit elements of gay sex, and the questionable maturity of the very flawed (i.e. human) protagonist; and you film the story in an art-house style; then you might get something like Happy Together.