In low light I regularly use two flash heads for my bird pictures, and find that they very quickly get used to it, I agree that it is more likely the mirror noise that spooks them but in my experience they quickly get used to this too. Using two flash heads allows you to create more natural looking pictures with greater depth than using a single flash,. The picture was taken with one difused camera mounted flash and another at 90 degrees to the right which was not difused.
