I remember reading the liner notes of an old Free Reed english concertina LP, Gordon Cutty I think, who played EC in a dance band in the north of England from the 1920s. He reckoned to play octaves a lot, often playing the violin part of the band's arrangements, because it was so easy on English. Personally I've always found playing octaves on an english very difficult. Maybe because I don't practice scales. It's much easier for me on the anglo, but then I usually play in the keys the instrument was built in.