Note that they may not operate on the same frequency. T-Mobile's coverage is exclusively 1900MHz. However, Cingular operates in many areas on the 850MHz band. As far as I can tell, Chicago is one of those areas.

850 tends to have better building penetration.

The 850 band goes back to the original analog wireless system, where there were two carriers licensed, the "A" and "B" sides. Through various mergers, Cingular has some of those original licenses.

The 1900MHz band was added later, and was opened to new entrants to the market.