Are two melody strings louder than one?
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Depending on string strike timing/forces used, it's all about the waveform spectrum and the phase interplay between the pitches & the harmonics produced from this. Even slightly out of phase components between the two will cancel slightly. Conversely in-phase will reinforce, the end effect is an altered sound spectrum which won't sound like 2 pure pitches played together. This the "fullness" that is heard but not louder.
Interestingly because wave prorogation/ pressure level is governed by the square root rule to make your Dulcimers twice sound as loud as all the others you'll need to strike the strings with 4 times the force of the others players.
I think slashed fingers/broken strings would be the order of day if you tried that