I have heard that some non-trinitarian Christians reject the concept of the trinity and say that there is only one indivisible divinity. Are there any Christian denominations that believe in a "duality", i.e. that there are exactly two persons in the god-head?
Furthermore, any Christians who subscribe to the Athansian Creed will also state:
The groups that do not adhere to the Athansian Creed are what are often called the "Oriental Orthodox," meaning things like the Syriac church, the Chaldeans, and various Eastern Orthodox churches. Percentagewise, these are very small, and their quibbles are less about the Trinitarian nature than about Christology.
To find a Christian denomination then that is non-Trinitarian, one must specifically look outside of the "mainstream" church. All Catholic, Orthodox, and Protestant denominations nominally hold this. (I say nominally, because many Christians are in practice bi-nitarian, neglecting the Holy Spirit - but if you go by profession, they are Trinitarian).
Your best bet for a dual-Godhead would be the Mormons. They believe that Jesus and God the Father are separate entities altogether. Jehovah's Witness are monotarian, rejecting the divinty of Jesus altogether. Finally, Unitarians, as their name implies, reject Trinitarianism. That said, many Christians would not include these groups within that definition.
There are Christians of all types actually. There's the trinitarians, the binitarians, the unitarians, and the modalists as far as the subject of the godhead goes. The term "Christian" is not based on any one specific of these terms or concepts. It's completely false to say that someone is not a Christian because they do not ascribe to a specific one of these concepts. Although most of the time it's easier to find the trinitarian denominations as these are the ones who really try to force others to believe as they do. Denominations who hold to any of the other concepts are not nearly as militant and are therefore not as obvious to perceive as the trinitarians. However, always remember that it is being saved by the blood of Messiah that makes us Christian. Not a specific concept of the godhead.
The Armstrongites believed that there were two in the godhead and some Anabaptists are modalists.