Cup, Chappy, and Lulu are right next to each other, with only a half a foot between them. BUT, since Chappy's tank is round, they rarely get a good look at her. i see little, if no flaring from them, not even Cup, who's a little flare monster.
my other three close tanks, are Theo, Zidane, and Ichi. them, however, i'd have to put borders between. Theo doesn't give two cares about neither boy, but i did notice alot of flaring from Zidane, and while Ichi doesn't mind Theo, whenever Theo gets a glance at Ichi, since he can't really tell what Ichi is, he wiggles excitedly at him, and i think that constant attention stresses Ichi out a little bit.