Complex sentence: In parallel with this Wagner started work on a fair copy of the score on 15 February, a task he completed on 26 September 1854, by which time he had also started work on the sketches of "Die Walküre".
Split into simpler sentences: In parallel with this Wagner started work on a fair copy of the score on 15 February. A task he completed on 26 September 1854, by which time he had also started work on the sketches of "Die Walküre".