Reading someone’s comment on one hand playing
I was just wondering if it is possible to play two wistles (or tabor pipe) of different keys at the same time. Maybe a third or a fifth apart in scale. You would have to get the back pressure right so they would both change octaves at the same time, and you would have the “same” notes, but it could be a cool effect. So what did I find?
Double flageolet

Dan Bingamon makes a double barrel whistle and Tully whistles has on that has a double with one drone and then a regular D whistle.
FYI Tully’s are out of production until 2007…
I saw Cathal McConnell play two Generation type whistles simultaneously. I seem to recall that he taped the top three holes of one, and played the top three on the other. I’m pretty sure they were in the same key (D?) Very cool indeed!
Some part of me knew that.
I’ve seen John Williams (formerly of Solas) do that, too.
I did a double flageolet recently for someone. The top two hole holes on the right hand whistle get normally open keys.
If you notice the double flageolet in the picture, there are little levers that cover the fipple windows - the are called “windcutters”. They allow you to silence one of the whistles.
Instead of windcutters, I put keys on that cover the fipple window.

Doing a third of fifh apart will work if you have very very good breath control. Also, the backpressure on one of the whistle fipples can be varied a little to help it cooperate more.
Here’s one with three low note holes, three high note holes and a drone - a triwhistle.

To the trekkies out there, I got my triwhistle layout idea from the Enterprise Nacelles.