Complex sentence: Districts 1--4 divide the city into four districts, and the districts 5--7 divide the city into three districts that overlay Districts 1--4, so every location in the city falls in two districts, one from each set.
Split into simpler sentences: Districts 1--4 divide the city into four districts, and districts 5--7 also divide the city into three districts. Hence, every location in the city falls in two districts, one from each set.