OK, it’s obvious from other posts that there are quite a few people out there who play both whistle and recorder so I’ll ask the question. How do you do it?!? I bought a sopranino recorder after I heard Nick Parkes busking with one in the street. Every time I try to play it I automatically revert to whistle fingering. How do you get your brain to handle the separate styles?
Practice.
Maybe it’s harder going from whistle to recorder, but I played recorder for a decade before picking up a whistle 2-3 years ago. While I still have trouble with whistle immediately after playing recorder (especially with the C and the right hand pinky), the whistle finger comes back to me in a few minutes.
JB
Play certain tunes on the recorder and other, different tunes on the whistle. Playing music requires a certain amount of auto-pilot - this is where practicing to get the tune in your fingers comes in. To play both the whistle and the recorder well, I keep their repertoire separate - works for me!!
A hearty “Amen” to the main thought here: practice! For me, since I’ve been playing recorders since 5th grade, and whistles around a year, it’s a little easier to keep my technique straight. And, to make sure I don’t neglect either one, my whistle stand includes four pegs to hold my tenor through sopranino recorders. (If I want to mess with the bass, I just get it out - it’s safer snug in its case…)
And, it’s an excellent suggestion to play different tunes on different instruments - helps the ol’ noggin sort things out. I do one solo where the middle verse is in Eb, and that’s when I switch from alto recorder to my Eb whistle, with the last verse on sopranino. Fun stuff! 
Did I understand that you started with sopranino? If so, you might wanna tackle either soprano or alto first - learn the instrument on one that’s a little easier, and then move outward, either higher or lower. (bass recorder is a lot of fun!!!)
Enjoy! There’s a lot of interesting possibilities with both instruments…
Cal
Let me be the first to admit that I couldn’t learn the whistle and play the recorder effectively. I had to set the recorder aside entirely and suffer through relearning. There are still tunes that I know I know, then find out that I know them on recorder, and need to relearn.
When I get together with friends to play the baroque stuff on the recorder, I’ll probably have a rough time swapping back.
I must say, it does my heart good to know other people have the same problem switching between whistle and recorder – misery does love company sometimes. In all seriousness, having played both instruments in the same band, and still getting confused between the two after having played whistle for 20 years and recorder nearly that long, you have to make a conscious effort when you switch from one instrument to the other. You have to locate where middle D (well, high D on the recorder) is mentally, take a few breaths to let this knowledge set in, then play. Actually, middle C and middle D are the notes that are the keys to solving the problem; once you have them set in your mind you should be fine. But I’ll continute to make the same mistakes myself, I’m sure. And as tubafor suggests, play soprano before sopranino. There’s room for your fingers to move on the soprano recorder!
Tom Bingham
Treat the whistle and the recorder as 2 different instruments that are closely related. The basic technique is the same : move from your knuckle.
I found playing D whistle helps in the conversion process as it has the most fingerings in common with the soprano recorder. A G whistle would be analogous to the alto recorder in F.
Another thing I found helpful was that I think in terms of drone, key and tune more when playing whistle whereas for recorder (which tends to have more chromatic material on average) I tend to learn the piece on its own terms (which instrument it was meant for) and only think about ‘best instrument for the piece’ only with Renaissance and earlier pieces. In the latter case, usually an alto recorder in G works great for the majority of stuff and is also the recorder of choice for many performers delving into early Renaissance and earlier stuff.
Check out Philip Pickett’s playing (usually with the New London Consort). He uses many ‘ethnic’ or ‘Irish’ ornaments to achieve a ‘proto-European’ sound. Another player to watch is Pierre Hamon. Good examples of what I’m talking about are the New London Consort’s ‘Music from the time of Columbus’ on Linn CDs and Pierre Hamon’s CD on Opus 111 CDs. I forget the title but it’s the one where he plays ethnic, medieaeval and 20th C Japanese music.