Mixed, compatible tunings.
General mountain dulcimer or music discussions
I've jammed with mountain dulcimer friends who use DAd as their home base tuning. (My home base tuning is DAA.) To me, the different voicings sound better than if everyone's tuned the same. Out of DAA, I'll re-tune to DGG, EAA; the keys of D, G, and A cover a lot of fiddle tunes. A Ddd or ddd tuning is really handy for covering 2 keys without re-tuning.
I love playing harmonies! When playing harmonies, remember that if a note doesn't sound right to your ear, you're only a fret away from a good-sounding note most of the time. To begin to learn to play harmonies, just play solid rhythm on the note that matches the key of the tune. More importantly, whether playing melody or harmony, it's the rhythm that holds things together-- it's not the notes. To practice good rhythms, put on any pre-recorded music you like, mute your dulcimer's strings, and strum with the music. Vary rhythms, skip beats, let your hand do whatever feels right-- it's more felt than thought. Kinda like playing air guitar. It's fun!