Due to a miscommunication, I have two of them coming. What do I do with them? How do you tune them? Are they only needed when playing with accordionists?
Yes! You play one of them, and use the other one to plunge into the bellows and rip them open. The tuning slide gives you more flexibility in finding exactly the right leverage.
Many, if not most, whistles are tunable to some degree. It allows you to make a fine adjustment to play with other, fixed pitch instruments. The whistles should be in tune with themselves if that’s your concern. They don’t tune over a wide range, just enough so you don’t sound bad when playing with a keyboard, cordeen, autoharp, or church organ.
Best wishes.
Steve
MT’s was the serious post. Mine was just joking around… (Works on bodhrans too)
Best wishes.
Steve
I usually play the lower octave with one tuneable whistle in my right hand, while I play the upper octave with another tuneable whistle held in my left hand. This works for me because I’m right handed. If you’re left handed just reverse the whistles. 
Also, you need a tuneable whistle if you want to play tunes. For songs, you need a songable whistle, aka a song whistle:
Of course, this one plays only American songs. If you want to play, say, Die Kindertotenlieder or the Kazakh version of Hotel California, you need an International Song Whistle. And don’t believe what it says on the box: “You can play any popular tune with only five minutes practice.” Yeah, right. I’ve been working on Coltrane’s “Giant Steps” for, like, forever. Maybe it’s not popular enough.
OK, Angelic Beaver, sorry … 
There are several reasons a tuneable whistle might be preferred.
You can compensate for temperature fluctuations: cold = flat, warm = sharp. As Steve Bliven said, in a group you can match other fixed pitch instruments which can’t tune to you. Or a group may choose to tune to other than A=440. Some sessions tend to tune a bit high. Or high whistles can tune a shade high for octave stretching.
A tuneable whistle should give you +/- 20 or 30 cents to work with. Many whistlers will tune to concert G or A - handy for one hand, and a strong mid-register note. Check the pitch against a tuner, or a tuning fork, or another in-tune instrument. Then adjust the head/tuning slide and your breath for good overall intonation throughout the range of the instrument.
Hope that helps, and watch out for those sneaky accordion players.
Cathal-inspired, eh?
Yes. Or this old thing: http://www.youtube.com/watch?v=gMqefRi_pXE
It only works if the two are in tune with each other but its twice the fun no matter how you look at it
So THAT’s why they make tuneable whistles!
Chang and Eng Murphy indeed…
Your question should be nominated.
