Complex sentence: The Wellesley Mk I had two separate cockpits, but this was changed in the Wellesley Mk II to a single-piece cockpit canopy covering both the pilot and navigator positions.
Split into simpler sentences: The Wellesley Mk I had two separate cockpits. This was changed in the Wellesley Mk II to a single-piece cockpit canopy covering both the pilot and navigator positions.