I'd recommend beginning at the beginning, so Volume 1. That way, you get the full story as the author intended without missing any crucial details or build-up.
I'd suggest beginning with the first chapter. It sets the foundation and introduces the main characters and the initial plot. From there, you'll be able to follow the story's progression smoothly. Also, check out some online manga platforms that have clear starting points and good reading interfaces.
A great moment is when they combine their Personas' powers. Imagine the unique abilities of Persona 4's personas merging with those of Persona 5 in a battle against a common enemy. It's like a display of a whole new level of combat strategy that makes the crossover fanfiction exciting.