I've got an ex-ambulance twin air-horn which I took off a dead ambulance a few years ago. I removed the rotating dee-da dee-da disc inside the compressor so it blasts out each horn normally.
I fitted the compressor where the ABS unit would be and the horns under the front edge of the tank. There's about a one second delay as the compressor winds up so I've still got the standard horn connected which works for short "toots" but for "making a statement" I hold the horn button on longer and watch blind car drivers jump out of their skin