Hi there!
I saw on whistleanddrum.com a low whistle/irish flute combo. The head of the whistle or flute were interchangeable on the same body. I was wondering if these were any good. The one I was looking at was the Tony Dixon low D made of pvc. I listened to a sound clip of the low D and I thought it was nice but wasn’t sure if the flute would be any good. OR, does anyone know of a cheap low whistle I could buy? Just want some opinions…thanks!! 
You’ll want to browse the flute forum for postings about the Dixon set — and look carefully, because it is discussed, and the feelings were mixed, as I recall.
I also recall that Sweetheart has stopped producing combo low whistle / flutes because of unresolvable acoustic issues. I’ve owned neither the Dixon nor the Sweet duos, but I do think it would be a wonderful idea.