people i has read my handbook of my roland snif i like not handbooks.. but i found notings around this i have found. but roland corperations not write about this tricky thing ..but you know if you have setup you stick and record first you first track ..and have done with this first track and start a new track 2 is it layered or sound to sound in tape terms.. but how you do ask you. well it is easy you first strak is stored in you usb dongle as WAV but you can set this also in mp3 mode but my advice do it not.. then you start you 2 strack with play and press fast REC REC and you distplay counter start in recordmode after i done this have i test this tricky on my other roland bk-5 and yes it works also seems BK serie have sound on sound..how fun is this...and i alle time spit my sequencer mode lol..
whatever as you are a roland BK owner try it out for you self...also drum track is posible...
what i think roland like take eyebrow like do secrets in keyboards ..i know also other trick but this is a other story..
here you go http://18.104.22.168/deasy.WAV