"Miniature" concertinas were/are traditionally made just as the vintage ones where with the same types and arrangements of reeds, reedpan, chambers, action, etc.... just less of them and usually higher notes (therefore shorter/smaller reeds as associated stuff).   The only concertinas I know that have reeds on different levels are some Englishes that Bastari/Stagi makes (and not necessarily their miniatures). This layering makes the inboard reeds slower to respond and have a decided different