Do any Christian denominations believe in a "duality" rather than a trinity? 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?
Historically, there was a group of persons who claimed to be Christians, but denied the divinity of the Holy Spirit. They were derided by orthodox Christians as Pneumatomachoi (literally, “those who fight the Spirit”) or Macedonians (after the proponent of this idea, named Macedonius—no relation to the geographical region by that name).  

The Macedonians were apparently “homoiousians”: those who affirmed that the nature of the Son is “similar” to that of the Father (a position that Athanasius discovered was actually nearly orthodox, at least as regards the divinity of the Son).

Hence, this sect could be said to have been “binitarian” or “dualitarian.”