This wonderful piece from Cantata BWV 170 was arranged for organ-duet by Sybolt de Jong.
The score is available for friendly prices on the homepage of the De-Jong-Brothers: http://www.dejongdejongmusicpublications.nl/organduet-/index.html
First "Engel" played the secondo-part as a midi-file and then "Bert" played the primo-part together with the midi.
I attach the secondo-midi-file below.
Please enjoy this heavenly music!