RE how to play it: The Hayden is one of two duet concertina systems with a consistent, repeating layout pattern, so this should give you little trouble. There's a "Rosetta Stone," and all you have to do is memorize it and practice it until it becomes second nature. I've spent the last few years doing the same thing with a different concertina system that also has a fixed, repeating pattern, as well as an accordion system (Chromatic Button Accordion) that's the same situation, and it's totally doable, particularly for someone who has some piano.
As with all consistent layout patterns, there will be certain note combinations, or note phrases, that present challenges or awkwardness given the idiosyncrasies of that particular pattern. You just arrange around it or "cheat" some way. Hayden people will be able to alert you to what they are.
RE arranging pieces of music on your Hayden: Just vamp the left hand the way you do on piano, possibly with fewer voices per chord until you, One, get the hang of the Hayden layout, and, Two, decide how many voices you want in your concertina bass arrangements. I like to keep them light rather than heavy because I like concertina to have a more prominent melody side and less bass noise, but with a duet, the sky's the limit and you get to choose how you like.
As a practical matter, on the "balance" question--i.e., not letting bass/harmony overpower the melody side, your one way of really controlling that on a free-reed instrument , is to play your bass/harmony effects SHORTER, more staccato and less legato, and release them faster. You'll see what I mean.
Congratulations on your new concertina and your new adventure. You and the Hayden seem like a wonderful fit given your piano background.
Edited by ceemonster, 26 January 2018 - 09:55 PM.