The trigger had a selector switch within the trigger guard. If one selected the right barrel to fire first, then a second pulling of the trigger would fire the left barrel. But if one selected the left barrel to fire first, the left barrel could be fired with the first pull, but the second pull would not fire the right barrel. Then, to fire the right barrel, the selector must be set to "right" and then the right barrel would fire. Not a real popular design feature.
I found a 2007 post on DoubleGunBBS from Robert Chambers that the single trigger was George Cline's patent # 748,263.