"...Could firing the top barrel so much disengage the sear on the second barrel and it just went off?"
Each time you opened the gun to reload the one bbl you did fire, the mechanism should reset the unfired hammer on it's sear once again. So both hammers are freshly set so to speak onto their sears each time the gun is opened and then closed regardless of wether one, both or neither of the bbls is fired.
That is if the gun is opened completely,,and assuming everything is in correct mechanical condition.
Trying diagnose what went wrong from a distance is impossible, but I'd suggest having someone who understands the workings of the gun take a good look at it.
Being a new(er) gun, the original mfg'r is a good bet.
There're so many things it could be. Plus you always have to figure the possibility of human/shooter intervention,, wether you believe it happened or not.
Having the gun looked over by the mfg'r or an excellent gunsmith can at least elliminate any mechanical possibilitys for the mishap.
