Maccann system duet concertina works for me. But I had enough of a background in music to understand chord sequences, bass lines etc. so I got on with the duet OK. If you don't have that you might consider:
- a continental chromatic accordion, or
- a cheap Hayden system duet concertina.

Both of these have the advantage that you can try a song in different keys and the fingering will be much the same (a bit like a guitarist using a capo).