so you want to do the old mercedes benz approach, they used a two way rocker switch on the dashboard to choose the city horns or the autobahn fanfare horns.
Get a switch that only locks on two positions and at any given time one remains on - route the horn trigger wire through this button and install your other set on the car along with a relay to switch to fanfare horns, the city horns can be left as is and the switch just routing power to its wiring as it is.