“Yes, of course.”
There you go. Finally! 
I am surprised no one has yelled; “April Fools!!”.
Not I! 
Why on earth would anybody do that?
You know guys, I’m frankly surprised at you. You all go “Whaaa-ha-ha-ha!! What a lark!” As if MTGuru didn’t have the idea of the century. Well, it’s perfectly feasable. Only slightly differently from the way he’s gone about it. What you do is get a D whistle, get a C whistle, get a lenght of tubing slipping over the ends of both of them, drill some tuning holes into the extension tubing, and hey! presto! a combination C/D whistle. Simple. I think the idea has uncounted possibilities. In a pinch can be used as a long blunt weapon in a pub session when things get a bit out of hand.
(It’s true about the real feasibility of such a beast, though…)( technically speaking…)
Oh, it was just a cruel joke… 
Philo
Okay for you. As for me, my horizons have been expanded, if a bit beskewed, for it. It’s sort of like a Möbius strip, or a Klein bottle.
Speaking of Klein bottles, here’s one made of Legos®:
How does the Klein whistle sound?
I dunno. Um…infinite?
It takes you outside of yourself!
Sounds messy!
You know, this could have groundbreaking medical applications. I’m expecting the Nobel Prize guys to knock on my door.