There are a few still doing duals. I don't do it as much as I used to - mostly I work in a band with three horns but even then I work it in on a couple of numbers. Its hard to do effectively - everything has to be balanced for resistance so both horns will play. Intonation is beyond tricky. You have to know music and chords, and you're playing in two keys at once. My learning to think in concert came in very handy for this. And don't forget, the right-hand one you're playing with the wrong hand. You only have the upper stacks and the palms, mainly on the left-hand one. Finally, you never set up a horn just to do duals - you still play each horn separately so they have to be viable on their own. This means the set-up on each horn has to be great singly and it has to work with the other horn perfectly too. I have always used mouthpieces of the same exterior design - for example two Level Airs or two Guardalas. Generally the mouthpieces have to be physically small for two reasons; the embouchure area is small, and the two pieces also have to be almost parallel if you're a duals player who has the horns close together. For that reason you typically will arrange your ligatures so the screws point in opposite directions or else they will get hung up.
There's an awful lot more to it but in theory you could play any line by substituting, and there's always harmony possible on any chord. I saw a guy in 1967 play 'Night Train' on duals and how he did that long descending phrase was incredible. It has to be done with substitution as I said because you only have those upper stacks unless you get into modified key work. I was never interested in modifying my horns but I think a few have done so to get more range. I mean one note would make a huge difference in what you could do.
