Ask then separately how they would describe the relationship with the other and what metaphor, analogy, historical incident and even cliche would describe how they relate to each other. Where did they meet, what was the common spark that drew them together, what was the piece of music/band/song that kicked of the duo. Can they act that out? Who is their fantasy musical lovechild? What is their fantasy job outside music? Also the standard approaches to brainstorming can produce some hilarious results e.g. Wat does this band have in common with a block of cheese etc. What do they like about each other and what do they hate.. but be careful not to break up the band now!