Duet concertinas, like Anglo concertinas, have the lower notes on the left and the higher notes on the right, with some overlap in the range available on each end. Like English concertinas, they play the same note on both the push and the pull of the bellows. If your goal is to play both melody and accompaniment, a duet concertina will do that more easily than either Anglo or English.
Hayden Duet
The Hayden is the most user-friendly of the duet systems. On a Hayden duet concertina with at least 46 buttons, the scale pattern is the same in as many as 8 keys, and chord patterns are likewise consistent. Also helpful for people playing in common fiddle keys, the buttons you’re likely to use most frequently are relatively easy to reach.
That’s all good. In the minus column, instructional resources are scarce, and while instruments are available at a range of price/quality levels, the choices are markedly more limited than for Anglo or English. At the lower end of the price range is the Concertina Connection Elise (note layout) a 34-button instrument that plays in a limited number of keys. Further up the scale is the R. Morse & Co. Beaumont (note layout) with 52 buttons. Between those two, the Hayden duet you’re most likely to come across is the the 46-button Stagi.
Note that “Hayden” is a system, named after its creator, Brian Hayden. It isn’t a brand belonging to a particular maker, so you will find Haydens under several contemporary labels. You can find nice vintage concertinas in older duet systems–Crane and Maccann, primarily–but none have keyboard layouts as logical and accessible as the Hayden’s.
Beaumont Hayden duet concertina by The Button Box.
Elise Hayden duet concertina by The Concertina Connection
Introduction to Concertinas and Accordions 