Spoiler alert
My "reunion" with Sadie ending was having her breakup with you after Cindy hints to her that MC cheated with Cindy. There'll be a beach scene where Sadie tells you about her past relationship with a dude that cheated on her with her bestfriend. She asks you something like did you ever really love her. Im thinking if you said yes, and then go through the rest of the story, where you fuck Jennifer (from Inside Jennifer), eventually you come to a point where you're talking to Jennifer on the phone, telling her you no longer want to be a manwhore, and break it off with Jennifer. Then you eventually bump into Sadie in a park, where the scene of how MC and Sadie first met is kinda recreated, with it starting to rain, and you both go under a tree where you both give eachother a peculiar look of wanting to get back together. I don't know if you'd still get that last scene if you told Sadie you never did love her.
The other "reunion" ending was reuniting with Mia, if you completed the full, pure Sadie loyal route. I can't exactly remember every step that I took, but throughout the game, you and Mia start working on becoming just good friends. You have to have told Sadie at the Halloween party that you are not interested in a 3some with Viv. Be as nice to Mia as much as possible, but always choose Sadie when given the option to choose between Mia or Sadie. Don't do any of the intimate stuff with Mia, besides the unavoidable sex scene early in the game. Which leads to one scene in particular I remember that pointed to becoming friends with Mia, was after the little cafe scene where Pete and Mia are, and eventually Cindy stops in after Pete leaves (the scene where afterwards you'd go either to the 3some or the boxing ring with Mia), if you did all the above (told Sadie you're not interested in 3some, and be nice to Mia while not doing any intimate actions), you'll end up going to the boxing ring, but instead of having sex with Mia, you and Mia just play around in a friendly manner. Ending should be you and Sadie living on your own, and Mia and her new boyfriend coming to visit you - hence "reuniting" with Mia I guess? But only in a friendly manner. This ending should also result in MC proposing to Sadie.