Traditionally, if more than one dizi were playing a piece together, it was likely to be either two dizi or three, if the dadi were involved. In the case of a two dizi set up, the usual set up is either two dizi of the same key, or one dizi (often a bangdi) and another one a perfect fourth (or sometimes a perfect fifth) lower.
So some combinations can include F bangdi and C qudi; two G bangdi. And if three dizi are involved, perhaps G bangdi, C qudi, and G dadi.
Under the traditional context, it is likely that the dizi are involved in either contrapuntal form or with some kind of call-and-response with similar parts but played in thirds. This is true whether or not there are other instruments involved. In this case, the manner of playing is similar to that of playing with a traditional Chinese orchestra, i.e. the need to pay close attention to intonation and how ornamentation is applied.
If the two dizi used are of the same key, it is important to ensure that their timbre are consistent unless otherwise noted; as for different key versions, if there is quite a bit of harmony playing involved, the timbre should have its noise reduced as much as it is practical without sacrificing what is unique about the dizi timbre.
Ensembles of More Than 3 dizi
At this point the ensemble would have gone from traditional to modern. While the dizi ensemble is, in many ways, inspired by the Western flute choir, there are some rather substantial differences that make it different.
The key difference is the issue of balance. In a flute choir, each flute (be it the piccolo, concert flute, alto flute, bass flute or even lower) have a compass of around three octaves, which means that the totality of the instruments readily span the entire grand staff. Even in situations where there are only three concert flutes and a bass (or say an alto) flute, the three-octave compass means that it is still possible to assign parts to the concert flutes such that there is a good balance of highs and lows (e.g. have two flutes play in the upper second octave and higher, and one in the first octave, and the bass/alto lower than the lowest concert flute).
For dizi, there are comparatively more instruments in the higher range (i.e. qudi and higher) than the lower (i.g. dadi or even beidadi), and that most dizi players are more comfortable playing the bangdi or qudi than the others. This means that from a practical perspective, it is highly likely to have more players in the higher range than the lower, resulting in a sound balance that is top heavy---this is true even with a large number of dadi/beidadi players since these instruments, while ``bassy'' compared to the regular dizi, do not descend much into the bass clef of the grand staff and therefore do not provide a ``true'' bass line.
Thus, when playing in such dizi ensembles, it is important for the bangdi and qudi players to control their dynamics well. One might go as far as to say that the bangdi should have less than half the number of members of the qudi and have the qudi players have multiple parts, keeping the bangdi focused on only being the top envelope/melody. At the same time, the ensemble will benefit from either having more dadi/beidadi players, and may even want to consider augmenting the ensemble with a true bass line in the form of a cello or contrabass.
More so than playing in a traditional Chinese orchestra, the dizi used in such ensembles cannot afford to have the dimo generate too much noise---the sum total of very buzzy dimo sound is a headache inducing set of inharmonicity that will mar the ensemble effect.