spitballing a few of your ideas together, next door/down the street neighbors since they were little? maybe their parents were close so they grew up basically as cousins and integrated into each other's families and then when the time came to leave the nest they just.... left together and became roommates? maybe they were just always thick as thieves and so it was natural to go to the same school or she could've gone to school and he could've chased a different path!