If the body of the horn grounds to the frame then you need to insulate one so it cannot touch metal to metal with the frame. This usually means you insulate the bolts that mount to the frame and make sure the horn body is not touching the frame. Rubber mounts should work. Your normal power line for the horn will connect to the insulated horn's power connection and a wire from the body of that horn should send power to the second horn that is mounted normally to ground with the frame.
You may be able to check the inside of the horn to see how it is grounded to the body (of the horn) or frame and dislocate that and run to the second horn.
I haven't looked at mine for a long time but as I remember it, I decided it would be easiest to insulate the first horn from the frame so it cannot ground, and run a wire from its body to the power input of the second horn.
I assume you have a power wire to each horn, coming from the same source, that is, they are wired in parallel.
