Years ago, most cars had twin horns - usually the big, old Lucas windtones with the big metal domes which give a lovely sound together. They were tuned to specific notes IIRC.
I thought standard MGR horns were loud enough already, especially when standing in front of them.
Doing a bit of digging on the OEM horns, the twin Hella horns 007-424 are 110dB but seems when they went to the single horn they also changed this to a Fiamm horn which is marginally quieter, and low note only.
https://www.hella.com/truck/assets/m...A_HORNS_GB.pdf
When I retrofitted twin horns to my '05 plate I used the twin Hellas as standard.