Duet concertinas have buttons arranged in 5 columns (Crane) or 6 columns (MacCann). There are also Jeffries and Hayden Duet systems, but both are vanishingly rare. All duet systems play the same note on push and pull strokes.
Prices will range from £600 to over £3000 - the basic rule is the more buttons the better. As always, the 8-sided Wheatstone Aeola is probably the most popular and the most expensive - and rare finishes will make them even more so.