For example: in this panel. I placed one balloon behind the other...then both behind the speakers heads. In the context of the story the words of this conversation aren't what's important...it's the interaction. It's merely to show two characters bickering, as usual. And what do people do when they argue? They talk over each other. They interrupt each other. And they don't really pay attention to the other person's opinion. Which is why I think the balloon placements really work in this panel! I also like how placing so much of the word balloon behind Chopper's head, making it almost impossible to read what he's saying, makes it seem like he's walking too fast for the balloon to keep up. His words are throw away. They are behind him...yet still dominating Glory's.
In this panel placing the word balloon behind both character's heads serves another purpose...secrecy. By being slightly obscured it offers the feeling of hushed voices, speaking behind someone's back...which works for this panel!
I'm not sure how many other times I've done this in my story. I'm suddenly conscious of the device and how it can appropriately and inappropriately be used in comics storytelling.