Here’s a map I published in my book Manchester A Football History. It shows the spread of support for both City and United across Manchester as recorded in an academic study back in 2002 (using data from 2001). Basically this was based on season ticket holders in each postcode area of Manchester and shows whether a postcode area was Red or Blue at that time. You can see that back then City had more season ticket holders in almost every district of the city centre and east Manchester while United tended to have more in western districts. However, this was based on the number of season ticket holders in 2001. Since then of course the numbers attending games at both clubs have increased significantly.
In my chapter I made a comment about how this analysis might change:
Gary James' Football Archive 