I think the big kids get a kick out of when I say things like "spend some time with your buddy" or "entertain your buddy for a minute". I see potential for the buddy system working well in another year when both little girls are mobile and we are out and about, away from home.
While this may change over time as personalities change, right now Jonah is drawn more to Greta and spending time with her. Since Gianna's birth, Lily has been little mother to Gianna and is really good with tending to her little chick. It's not that they don't get along with the other little girl, it's just that right now they have their buddy.
I try and make sure that there isn't any segregation or indifference that keeps them from going beyond just spending time with the one they get along with best. With Jonah being the only boy, he needs the extra encouragement to care for his sisters and try to understand them rather than be frustrated. For instance, I encourage Jonah in learning how to handle Gianna even if they are too similar in personality that they can't be left alone for too long.