Back in the day (high school!) I fitted a set of air horns to our 240 wagon. I kept the original horns activated by the steering wheel switch, and I used one of the older-style rocker power window switches in the centre dash console to activate the air horn (with a relay as the air horn compressor uses a fair bit of power). I found that when you really NEED the loud horn, you fumble around to find the switch and it's too late. If I were to do that again (dual horn types) I'd put a 2-position toggle switch on the dash and have it always set to activate the LOUD horn via the steering wheel. In the case where I might want to give a gentle "toot" I'd flip the switch and use the steering wheel horn button to give the toot, then switch back to the loud horn as default. Just a thought. Regardless, if the loud horn draws much current, you'd be better off to put a relay in the circuit. TBH you rarely need a quiet horn, so if it were me I'd just put the loud one in place of the existing horns and if need be put a relay in to be sure. You can do a very quick press on the horn button to get a "toot" even with air horns, which is what I have on my current 240 wagon.
On the 240 EV, I've retained the existing single (1993 model) toot toot horn because I sometimes have to use it in car parks with pedestrians not paying attention, and I don't want to blow their brains out with an air horn. :)