I have a small group of friends that gets together to sing and laugh and we have guitars, spoons, and a box. I’m thinking of adding a flute, whistle, or both to the mix but was unsure of what would be the best mix for jamming along. I have a Irish flute nothing fancy just $200 DD one and have been debating on purchasing a whistle. I’m looking for some advice on how to pursue this.
Thank you all for your time and patience.
~Dok~
Yes, purchase a whistle and go for it. 
Depends what key you usually play in to some extent and what sort of music you are playing, but my recommendation would be for a Dixon Trad in D, or something equally cheap and cheerful.
I’ve read that having one whistle in D and another in C is a good combo for most playing is this true? And what of my Irish Flute? I’ve also read that I should master the whistle then go on to the flute because the fingerings will be the same. Thanks for your quick response you guys! 
Are you referring to the Tony Dixon brand?
And if so should I look to a High D or Low D not sure if a lower whistle or something more like a penny whistle would be more appropriate from the videos I have seen the Low Whistles appear to be more versatile and have 2 octaves…thoughts?
A low whistle is more of an investment, so I’d start with a high whistle in D and see how you get on with that. (The guitarist can always pop a capo on and spoons play nicely in any key). A whistle in the key of D will play tunes in D and G easily, not to mention Em and Bm too.
Yes, Tony Dixon. Metal, rather than polymer.
I like instruments that are tunable when I play with other people. Regarding the combination of instruments, play whatever you have available. Different combinations of instruments and people make for some interesting interpretations of songs.
That’s certainly true if you are playing the Irish traditional dance tune repertoire. The D whistle is the standard whistle to match the D pipes and flute, and covers the prevailing tune keys and modes. The C whistle covers the less common keys, such as D Min/Dorian, G Dorian, C/F major.
But if, as I’d guess, you’re playing something else, then all bets are off. You collect a set of whistles in various keys, and/or adapt what you play to the whistles at hand.
High and Low whistles have the exact same capabilities. Both can play 2+ octaves.
Yes and no. The whistle and the flute finger identically. If you’re still at the point of learning fingerings, whistle lets you learn the fingerings and start to play out while you’re still developing a good flute embouchure, which can take a long time. But at some point the flute and whistle diverge considerably in terms of style and technique. There’s no reason not to play both, but they are different, each with unique qualities.
As Hoopy says … You’re comparing a choice of a good low whistle costing perhaps hundreds of $$$ with a standard D whistle, a good one which can be had for $6-30. Regardless of eventual preference, acquiring a D whistle is rather a no-brainer.
The issue with transfering between Flute and Whistle is the main reason I am considering The (Tony) Dixon Duo. This way you can play either with the change of the head.
Myself, I haven’t been able to play flute heads. This is why I picked up the whistle. Mastering the whistle head takes much longer than getting it to make sound. So, by buying a duo you should be able to get started quickly, while keeping plenty of chalange in front of you to master the two head. You will also get the bonus of versitility.