Marpurg's chorale prelude on the choral "Christus der ist mein Leben" consists actualy of two seperate pieces, rolled into one composition. You get, as it were, two preludes for the price of one. The first has the chorale melody in the pedals, while the hands play a joyful accompaniment, full of triplets and syncopations. The second one treats the choral melody in canon between the right hand and (again) the pedals, while the left hand plays an accompaniment in eighth notes.
Both pieces could be played seperately in their own right. In my performance I play them as Marpurg notates them in the original edition: as two halves of one composition.