No wonder everyone has more than one.
General mountain dulcimer or music discussions
When i used to go to old-time music festivals and gatherings a lot, I'd often bring two dulcimers or two banjos:
One dulcimer I'd use for the keys of C and D (tuning up or down one step on all strings), and the other dulcimer I'd use for the keys of G and A (tuning up or down one step on all strings).
I'd often do the same with the two banjos.
Why? Because going between the lowest key (G) and the highest key (D) sometimes means retuning a string by four whole steps up or down, and that's a real big jump which can result in more stressed and broken strings if you do it often. It 'can' be done, but is frustrating to do back and forth in the setting of jams or if folks change keys too often. The alternative was to use capos, which I try to avoid because I dislike the sound and feel.
Sometimes, to avoid bringing four instruments, I'd bring a dulcimer for C and D and a banjo tuned for G and A.
One minor point- I'd like to mention that beginners often think that DAd is a completely different animal than CGc. Or that DAA is a completely different animal than CGG. They balk at having to 'learn a different tuning' and all that entails. They don't realize that it's really the same tuning but simply one whole step up or down, and it will play in the keys of C or D. That means they can use all the same fingerings and frets in the key of C as they are already familiar with in the key of D. They can follow the same Tab numbers, and don't have to learn new stuff. They'll just be playing in the key of D or C. Same is true for the (1-5-8) tuning pair of DAd and CGc. If you are comfortable playing in DAd you already know how to play tunes in CGc... Likewise for the 'pair' of (1-5-5) tunings DAA and CGG.