I think that it might be an error to have a double boat horse as well as the fly boat horses: from what I recall of my research, fly-boats were the only time that teams of two horses were used. I post here rather than in the bug reports section in case there was some historical precedent used for the two horses of which I am unaware; might I ask - is there one?
I'm relying on your research here - obviously I misunderstood it.