Oh shoot - there's this company that sells a very directional horn that you install behind your grill - it's like 2 big speakers. Puts out some sick decible level too (125-150db?) - something loud. I remember a story of this guy stopped at a traffic light doing a horn comparison with an ambulance (not on a call), it was comparable!

Otherwise for a dual note, I agree with the above - buy a $10 cheapie and experiment. As a music guy, I know 2 notes combined at different pitches can have a variety of possibilites - some might sound nice (like a 5th or octave) and others might sound nasty (like a 6th that is slighty sharp, or any note that is not in tune with the other).