One of the assumptions I make as GM is that the players should always be in the spotlight. Sometimes the players find themselves in a situation where they watch NPCs speak to each other. I don't like using this kind of situation, because it forces the players to watch me talk to myself and that turns them from active participants into an audience.

Usually I handle it by avoiding the situation. I try not to write scenes that the players watch. Instead the players speak to the NPCs one at a time. When I do come across an exchange between NPCs I opt to summarize the conversation instead of acting it out. 

I don't like this limitation though. It gets a little contrived when all conversations in the game feature 5 PCs and 1 NPC. Any suggestions? Either how to avoid doing it, or how to effectively role play a conversation between two (or more) NPCs that the characters may or may not be interacting with? 

How do you keep clear which character is speaking? How do you make sure that the conversation feels like a real one and not like one person playing both roles?