A question about strings and placement

I am fairly new to the building process of dulcimers. I have built 6 stick dulcimers and I am gearing up for a mountain dulcimer in a couple of weeks.

My question is this.. 4 strings?.. looks like the two small strings are tuned to the same note.. is this for more volume of sound? Does it mater how far apart I space these?

Also, a question on scale length.. I have been building with 22.75" length. I know on Stew-Mac's website I can punch in any length and get the fret spacing. Is there any benefits to a longer scale? More sound, a different sound.

Thanks everyone and merry christmas!