The Uzbek dutar is usually tuned in the four types of tunings: 4ths, 5ths, unison or octave. My semi-educated guess is that tunings in 4ths and 5ths are the commonest ones, with the latter two tunings used only occasionally.
Uzbek dutar belongs to the type of dutar that is ‘polyphonic’, which means that both strings are stopped when playing, as opposed to, for example the Torbat-e Jam dutar, in which only the first (higher) string is stopped, with the lower string providing a drone. One would think that the music played by the two types of dutar would be very different, given that one type is polyphonic and the other is monophonic. This is primarily because the word ‘polyphonic’ is somewhat misleading in this context.
The dutar is essentially a melodic instrument (what instrument – save percussion – isn’t melodic in music from this region?). What unites both types of dutars is that the melodic line is almost exclusively played on the high string, whilst the lower string provides accompaniment. This means that, the melody is played by moving up and down the neck only, without any string-crossing. This accompaniment is either the drone, or various ‘harmonizations’ of the melodic notes, depending on tradition. It appears that the harmonizations are, most the time, not composed into the melodies, but added for euphonic purpose. Of course, there are conventions as to what kind of harmonizations are agreeable and which ones less so (ie not used). This is what I learnt when I realised that a tune that my teacher played me, which I thought was familiar but whose harmonization I could not recognise, turned out to be a reharmonization of a piece I’d played him the day before when we first met. More about that later. The most important point I want to make here, is that in learning dutar music of any sort, what is essential is the melody. It is the melody which defines the music, far more than the harmonization. As a guitarist who learned pop songs mainly by their chord sequences, that necessitated a very different way of listening to the music. Choosing to play a song with the string tuned a 4th or a 5th apart then depends on which would facilitate better-sounding harmonizations.