I highly recomend reading the novel "Holes" (the 2003 film is probably the only film to break the "book was better" rule, being 99% faithful, but I still recommend a read through). Long story short, the novel basically has three stories running through out, and the first starts in media res and the "inciting incident" of that first story is told concurrently with the present day first story and flashback second story. Later a 3rd entirely seperate flashback story expands upon a minor plot point mentioned in the begining.
With In Media Res stories, your "Inciting Incident" is not going to be the backstory, but the events that happen as a result of the heroes meeting up. Consider Star Wars (A New Hope) which opens with Vader's ship attacking Leia. The inciting incident isn't "Leia has the Death Star plans" and we don't get to see that story for nearly 40 years. Rather it's Luke receiving her "Help Me Obi-Wan Kenobi" while repairing R2-D2 (and they are focused on so heavily because at the time, George Lucas was homaging the Japanese film "Hidden Fortress" which is an epic Jidi Geki (lit. Period Piece, basically a Samurai Film. Also the origin of the word "Jedi") told from the point of view of the comic relief duo.).
Another film you should look for inspiration is Rashamon (also a Japanese film, but I you've probably seen the basic plot... Hollywood loved this film). Here, the story starts almost at the end, with a priest and a wood carver, reflecting on a disturbing series of events that took place. A samurai was recently murdered and at the trial, three separate witnesses tell an incompatible tale of events leading up to the murder. Not unusual accept each witness confesses that they and they alone killed the samurai (and one witness is supposedly the spirit of the samurai himself!).