I fitted an extra horn to my 1200, no problems apart from physically where to put it. No fancy wiring required, just connect it in parallel to the existing horn.
However, there is always the chance that some horns will exceed the current draw allowed by the circuit, resulting in the circuit being turned off by the control gizmo (technical huh).
Steve,
At the risk of sounding IAM ishmate. Under what sort of circumstances would you use the second horn. Surely if things are looking iffy, spend the time looking for the horn button getting on the brakes.
Good for making pedestrians jump in town tho!!!