Used to double-leg from DTW or LGA when I was based at either of those bases and there was no chance of catching a direct flight. Worked great each time with a huge number of options. As previously stated, usually the United and American flights would depart within 15-20 minutes of each other so you'd have to pick the one with the best loads. I always thought it'd be as close to an ideal commute as it can get (still sucks).