You should honestly just be able to choose which character will talk next unless it's a scenario where the other party demands that someone in particular will answer.
Lae'zel actually has one particular interesting conversation with fellow gith in which you can let her talk, or talk in her stead (at least if you're a gith). That's cool. But if it's a conversation between the party and someone else rather than between 2 people, anyone should be able to speak.
