Only one horn sounds when button is pressed but both horns sound when alarm is activated. Is there a way to get both horns when pressing the horn button?

 

Patch the other horn in or better still buy some air horns these days they are cheap.

 

Loud and not loud enough are totally subjective. IMHO the horn's purpose is to let the other driver(s) know where you are and not necessarily startle them into doing something wrong. Keep in mind there are 4 outcomes .. the other driver realizes their mistake and take corrective action, the other driver gets irritated/angry and keeps doing what they are doing, get startled and make things worse, or simply ignores your warning. Only 1 is desired outcome.

Also note that the sound intensity outside vs inside the cabin is different [sound being directional].

Granted more is better!

Salim