The coaches aren't separated because after champ select, they don't interact with their teams at all. I would imagine they sit together because they choose to watch the games together, bouncing commentary, commiserating, sharing the experience of the highs and lows with someone who also has no power over the situation after the pick/ban phase. The league pros have been around each other an awful lot, and friendships form. Friendly rivalries, good-natured trash talk, laughing at the silly misplays your team does, etc. -- all of these things make the experience more social and more enjoyable. I'm willing to bet that if someone doesn't want to sit with the other team's coach, they don't have to.