I wonder what their ignition system was? You'd think there would be a linked combustion chamber that should cause both to ignite simultaneously (the small central protrusion at the top appears to suggest that). Perhaps an intermediate detonation chamber, linked by small passages into each main chamber, to prevent blowback into the other barrel and ensure both balls were launched at a similar velocity. I imagine it would be pretty sensitive to variations in powder load and wadding between the barrels though.

Perhaps a better way would be to make a split ball with the chain either leading or trailing it in the barrel, where each half of the ball locks into the other with a pin or other protrusion (to keep them joined while in the barrel). Then when they exit they would be free to separate.

That’s cool