A dual-tone horn sounds much better to me because it forms a musical chord, rather than just a beep. As far as the acoustical physics of adding two horns output together, I would guess it is similar to audio formulas that I am familiar with. Doubling the audio power to a speaker will raise the output 3 db. Putting 2 speakers together playing the same signal should yield the same 3 db boost. Two horns should be the same as two speakers as far as output levels are concerned. Since the two horns will be playing different frequency tones, I don't know if the same thing holds true.
I'm a middle aged guy, with a middle school mind.