There's also 'Jujutsu and Bungo in an Alternate Realm'. This fanfic delves deep into how the powers from both universes would interact. It shows the characters trying to understand each other's abilities and form alliances. It's a long read but definitely worth it for fans of both series.
When writing the fanfic, focus on the character interactions. Since they are from different universes, their different ways of thinking and acting will create a lot of interesting scenarios. For example, how would Yuuji from JJK react to Dazai's suicidal tendencies? Also, make sure to balance the two universes' elements so that neither overshadows the other.