I do like adding strings to the low end of my dulcimers and recently have been having a lot of fun with D2 A2 D3 A3 D4, but on a 4 string dulcimer I much prefer A2 D3 A3 D4 compared to D2 D3 A3 D4. In other words, ADAd instead of DDAd.
I'm with you, Nate. That is a common octave mandolin / Irish bouzouki tuning (I also play mandolin and octave mandolin). It's great for flatpicking fiddle tunes in D on the dulcimer an octave lower than would normally be possible, i.e. it lets you "dip below" that low D string.
You might want to try tuning your two A strings down to G: Then you can play in G without a capo!
My two main dulcimers have five equidistant strings, with two unison high D strings. One is tuned A,DAdd, the other G,DGdd ("A," and "G," indicate that those strings are tuned an octave below A and G, respectively).