New York New York, Complex Recommendations
Both are emotional, heart-wrenching love stories that eventually depict the couples growing old together. There is also another part that is almost identical, but I'll let you find that one out yourself. :)
Manga about two men slowly falling in love with each other with some drama.
Both stories also have a similar ending
If you have read one of these excellent yaois, you should definitely check out the other. Each manga follows the journey of a gay couple throughout the entirety of their living relationship, start to finish; a wonderful rarity in the romance genre. They have similar atmospheres and other parallels in the departments of characters, family, and the final conclusion. So much so that it would be safe to assume one likely influenced the other. Complex spends more time on the couple when they are still young children, and New York New York has an arc that's very crime drama-esque, but they both are touching bitter-sweet