Mixed, compatible tunings.
General mountain dulcimer or music discussions
Skip, A simple fact is that you can't play a true mixolydian mode tune which uses the flatted 7th note in DAA which gives you the scale notes needed for Ionian mode tunes. DAd is the tuning needed for mixolydian mode tunes as the 6th fret will have the flatted note of the mixolydian scale.
Most people now who are using a DAd tuning aren't really playing mixolydian tunes which are actually fairly rare compared to Ionian mode tunes. With the addition of the 6+ fret a player can get the Ionian scale to start at the open position tuned DAd. Traditionally the Ionian scale started at the 3rd fret on a completely diatonic (no extra frets) instrument.
So my brother will play "Brother John" in DAd as 0120, 0120, 234,234, etc. while I play the same song in DAA as 3453,3453,567,567. The notes are exactly the same, DEF#D, DEF#D, F#GA, F#GA tuned DAd or DAA.
And it doesn't matter if you play noter/drone or chord the melody notes remain the same. The only thing that changes is where you find them on the fret board. So chord finger patterns change but a D chord is a D chord. There isn't even a change in the drones as the DA drones in DAd or DAA remain at the same tuning.
Some people though when playing chords or finger picking can find the drones irritating. I was actually told to my face by someone that she didn't like the drones. She was the type that wanted the group to only play chord/melody so that everyone would sound alike.

If you'd like to see someone playing simple harmonies/back-up, look for "I'm Troubled" and "Down the Road". I apologize for the examples being my own play yet that's all I know, really.