[Flat Ernie]

You can run the horns in series if one is electrically isolated. They will not be as loud as they could be. This is because they make/break each other's circuit, but it does help to keep the current down a little - start-up current will still be high though.

It is far easier to run them in parallel as original and have them be louder. If you still have a 6V horn relay, they are often better than 12V replacements because the contacts are often heavier. But I've found that aftermarket relays are better still.

The horns, which already draw high current, particularly on start-up, will draw double that on 12V.

I really like to find and use the 48-52 F1 dual trumpet horns - man are they loud on 12V. But most trumpet horns will be nice and loud. The other thing I like about the earlier horns is most are adjustable and the contacts can be cleaned. Pull the bell off the back and you can dress the points, replace any frayed wires, and adjust the tone/sound/volume.