Hm... I don't know. I'd sort of seen them as being roughly equal. Like, they both have their specialties - FI analyzing the evidence found, and D actually finding it - but when it comes to the middle ground of putting the peices together, they're pretty much even and work well together, bouncing ideas off one another.
Maybe they just have opposite stylzes of it. FI is very precise, very logical, and D has a good gut for it. As they work together, FI leanrs to trust D's gut, and looks to find ways that they link up together, while D learns that sometimes his gut can be wrong, and uses FI's linear train of thought to point him towards the right connection.
I don't know if there's a trope for that. Like, a power trio but without the superego. Just Ego and Id - instinct, and reason. Either way, I still ike the thought of them getting really close to a conclusion, and they just start shouting ideas back and forth in rapidfire, and nopony else can figureo ut what's goi8ng on but suddenly they both know the answer.