The only ones I sound together are ones on a manifold made for that. So my K-12 (bottom left in my photo) is sounded as two horns together because they’re on the manifold made for that combination, others are singles so I keep them that way even though the individual horns themselves are the same if that makes any sense?
Takes twice the air to run two at a time though, so it can get eaten up fast. I think a lot of people like the sound of a twin or combination horn better than a single as they can have a little more depth or complexity to the note. Kind of like listening to one tuba vs a group, there’d be more depth to the overall sound. I’m not an expert on them though, some of the real railfan guys have made a life’s work out of studying what gets that best sound.
From here yours looks like a die cast K-1? Should sound pretty clean