Another suggestion would be to go to a Concertina Festival where you will get a chance to hear and have a go at the various instruments.Another suggestion would be to contact Button Box in the USA and they will give you all the forthcoming festivals and if you pay them a visit will let you have a go on all the instruments.Or follow the suggestions already mentioned by Andrew.
A Duet Concertina Jeri, has the same note in and out (as the English)but instead of the notes being on alternate sides,the Low notes are on the left and the high notes on the right.This enables you to play the tune on the right side of the instrument and accompany yourself on the left.
As it is the same note in and out, the instrument lends itself to classical music and song accompaniment.A wonderful instrument if played well.
Good luck
Al