When choosing flutes for a duet, it is easiest to play flutes that are of the same key. But also some flutes of different keys work as well. The flute harmony wheel suggests additional keys that can be played together. Starting with the key of the lower pitched flute, one can move around the harmony wheel in a clockwise motion, either counting around 5 or 7 notes to find a flute of higher pitch that should play in harmony with the first flute. The spokes on the harmony wheel provide a short cut to counting around the wheel.
One way of harmonizing is to play the same thing on both flutes, but it can be more interesting to improvise a duet, where each player does there own thing. The trick is to avoid notes that donít sound good when played together. This may sound tricky, especially if you are not quite sure what the other is going to play next, but it is simple to accomplish by avoiding certain notes. If you never play those notes, they will never clash with the other performance of the other flute.
First, both players should restrict themselves to notes of the basic scale, since all those notes basically work well together. This works great when the flutes are in the same key. When the flute keys are 5 notes apart (which musicians call this a perfect 4th), the higher flute should also avoid the in the basic scale. When the flute keys are 7 notes apart (which musicians call this a perfect 5th), the higher flute should avoid a different note, the in the basic scale. Actually, if these two notes are played briefly such as passing thru them onto another note, they are not much of an issue. But they can destroy harmony, if you dwell on them to long. Otherwise, all the other note combinations should work just fine. With practice, one can develop an ear for flute harmony and know when these rules can be broken and when other notes can be used.
To give a specific example: if you have a flute that is in the key 'E' minor, another 'E' minor either the same pitch or an octave higher can be played together with ease. It helps if both players restricts themselves to the fingerings of the minor pentatonic (basic) scale. If you lookup the 'E' on the harmony wheel, you will find it also harmonizes with an 'A' or 'B' flute. On a higher 'A' flute, try to avoid the . And on a higher 'B' flute, try to avoid the . If the key of the 'E' flute is higher than the 'A' or 'B' flute, then the 'E' flute avoids the extra note as described in the prior paragraph.
Of course this discussion assumes the flutes in question are tuned to be played with other instruments. Some makers refer to this as being "concert tuned". Experienced players will actually test pairs of flutes to see if all the notes are in tune, before trying to play a duet.