My wife and I have been playing for about three months now and I’m curious about how duets are handled to provide more interest. We both have “D” whistles. What would it sound like if one of us used a low “D” while the other used the standard “D”?
I’ve heard this done several times and I think it sounds lovely. Also, flute and high D duets are quite common and sound very nice.
Though I’ve got to say that one of the most fun things in Irish traditional music is playing a duet with someone else playing the same instrument you are. Both play the melody together, and try to get your ornaments to complement each other’s. There’s a lovely wild joy in that sort of playing.
As long as you aren’t out of tune on the highest notes it’s a lot of fun on two high whistles.
if you’re reading sheet music, one of you can play melody, one can play harmony. one of you can play the top set of notes, one the bottom part. one of you could even play the bass line. i play duets with my wife but she’s playing a keyboard. she plays the accompaniment and i play the melody, usually. have a nice time. playing duets with my wife is great.
I would recomend just experimenting until you find something you like. The only problem I can see with a duet between a high and low D would be intonation issues (I say this only because I play the “modern” flute, and sometimes duets that are just separated by octaves can make you want to pull your hair out). But, if you can get the two whistles in tune, as gallant_murray says, I am sure it would sound lovely. 
I was just doing some recording of whistle, low D whistle, and alto flute, and agree that intonation is an issue. Even though I play high and low whistle with others (including guitar and dulcimer), hearing my pitch problems between two whistles was a little startling. It took a couple of hours in the studio to get it acceptable. The alto part, being in harmony, wasn’t nearly as tough as the unison (octave) parts. Very instuctive.
Hi John,
I like Colomon’s reply on this one. Playing the same tune with someone else yields a magical feeling of communication - listening closely and playing at the same time. If you are adventurous you might even venture into harmonies on a tune you both know well - from my own experience of a few instruments it is so much fun hunting for harmonies - try taking a single phrase and have one player play 2-notes up (eg if note is B, harmony is D. So a phrase BGEG would harmonise DBGB). The realy fun bit is when the 2-note-up harmony doesn’t sound right and you then have to hunt for a note that fits.