I’m looking for a low D whistle that can be changed into a flute by removing the fipple and replacing it with a flute head joint, along the lines of the Dixon “duo” model. Mr. Dixon’s is the only combo like this I’ve seen. Anyone know of any others? Oh, and pvc, please… 
The only other one I can think of is the Sweetheart Resonance. But it’s wooden, not PVC. Also, it’s expensive.
I’ve not heard flattering statements regarding the way the flute portion of Mr. Dixon’s duo plays. The whistle, from what I’ve heard, is fine.
Well, I got the Dixon Low D Whistle/Flute Combo. I’ve never got on very well with the Flute head, I have to say. Having said that I’ve not got on very well with any flutes.
However, I do like the whistle head. It doesn’t have the resonance of the Burke Aluminium Low D Viper I just lost (waaaaah!) but it’s a nice player and easier into the upper octave.
Sadly, the resonance low-D whistle/flute combo is no longer available.
There were a few problems making a body that brought out the best of both heads so the Sweets have stopped offering the combo. Both the Resonance whistle and the Resonance flute are still available separately.
Doc
To elaborate on Doc Jones’ post: We offered this combo and were real excited about it for a while. Then, I noticed that the body wasn’t quite swapable. The pieces fit up alright, and most of the scale was compatible, but the low D was flat on the whistle. Since then, I’ve changed the bore on the whistle (larger diameter in the tail) and the tuning is good. One of these bodies now will not work well as a flute. As a result, we don’t say that the bodies can swap. We now offer bodies intended for whistles, and different bodies intended for flutes (which look the same on the outside) and each gives a good scale in its own right. If a given head is played with the wrong body, it will still play, but not as well in tune as the right body. I’m not sure that another bore will make the swapping problem any simpler; l think these bores give the best intonation and response.
Thank you all for your imput! That sets it straight for me