There are various kinds of duos, the most obvious ones being those consisting of two singers. Good examples are the Righteous Brothers, the Everly Brothers, Simon & Garfunkel, Sonny & Cher, Hall & Oates, Sam & Dave, as well as Lee Hazelwood & Nancy Sinatra. There have been duos in all decades and all genres, including hip hop (just think of Eric B. and Rakim, or Outkast for example). A duo is basically the same as a solo artist, with the only difference that there are two of them. The rules remain the same. The problem with the duo is that it’s really hard to form a good one. Two equally talented singers with voices that work well together will have to meet, and they will also have to get along as human beings. Another kind of duo is the one with one singer and one instrumentalist, and in this case they will most likely also have a backing band. You may also call this a band with only two permanent members if you want. Think of Ike & Tina Turner, or the Eurythmics for example.
Then there are the quite unique duos, like the White Stripes for example. Jack and Meg White were married at the time, Meg was playing drums while Jack was doing more or less all of the rest, without relying on any backing band. Minimalistic, innovative, and lo-fi. It was unique, but now that it has been done I don’t know if something similar can (or should) be re-created. But they showed us that anything is possible, that the rules can be broken, and that you can be very successful with it. I should also name Daft Punk here, although they are a bit more like a producer duo that’s also performing live from time to time. Anyway, they too demonstrated that you can be very successful over decades by doing something that’s very different from anything else you’ll find out there.
If you want to be a duo, then the goal must be to create something unique already at the point of creation. No matter if it’s a special sound created by the unusual combination of two voices, if it’s the way you’re doing music, or if it’s your look, it has to be special and it needs to fit.