a person who likes the uke or seme in the first couple becomes an uke in the second couple. much like hiroki from junjou romantica or kashiwagi ayumu from interval

Yes, there is/are! In some instances, the heartbroken character gets his own spin-off or story, so you might need to read the main story or prequel (in parentheses), first:

Gravity Eyes (Tsumasaki ni Kiss)
Bitter Pain (Sugar Pain)
Kimi ga Koi ni Oboreru (Kimi ga Koi ni Ochiru)
Kimi to Kore Kara (Konya mo Nemurenai)
Renai Kyoutei Nukegake Nashi!
Saezuru Tori wa Habatakanai ("Don't Stay Gold")
"Bokutachi no Sotsugyou"

In other instances, the heartbroken character gets his own chapter or two within the main story or series:

Tachibana Sou from Innai Kansen & Shujii
Ueda Kyouichi from 1-en no Otoko & Limiter
Kumakashi Teruhiko from Sex Pistols
Sagara Yuuji from Bukiyou na Silent
Kitahara from Higouhou Junai
Andou from The Circle

This list is far from exhaustive, so I might come back to edit it, again, if other titles come to mind.

you are a godsend please do! <3 thank you!

Kijima-kun -- It's not obvious in the first volume, but from the second one onwards the person who had a crush on one of the guys from main couple gets his share of page-time with his own (future) lover.

And the son who had a crush on his father's BF in the title story of Kaerimichi de Koi ni Madou gets his own guy in Hatsukoi no Kanata.