I'll bite.

By shifting my 200K or so of travel between CO and AA, I get a higher possibility of upgrades on my principal route (the return of which is my Flyertalk login name.) if I fly AA. AA has a far higher upgrade level for an EXP to/from MIA than CO does for a Plat. I get VIPOW's from AA for travel to South America, these do not exist on CO. I also find that AA seems to release inventory for these upgrades VIPOW/mileage routes sooner than CO does for the routes for which you have to use miles and cannot obtain EUA's (business first routes). AA has far better service to the Caribbean. If you need to go to SDQ, as I do at times, last I checked, CO's service was from EWR, which is WAY out of the way for me and means two days of transit time.

OTOH, with CO I get better availability of flights and upgrades to Mexico and Central America from my home airport, SAT. All Y fares on CO are YUP's, at times AA seems to refuse quote YUP or KUP fares and only quotes Y26's. CO grants you double segment credit for H, K, Y and all first and business first fares. It is far easier to make top level on CO (90 segments) than on AA (100), at least if you live in Texas, because of the fact that WN forces its competitors to keep fare down, but you still get double segment qualification for the upper level fares on CO. On AA a segment is a segment and you need 100 of them for EXP qualification.

On the routes in which CO has to compete with WN, I can very frequently guarantee a seat in F through a YUP fare for a very reasonable price. For some reason, AA does not want to compete on price with WN on these destinations, at least for KUP and higher fares. The fares which get you into first automatically on the WN competitive routes seem to be much more expensive on AA than on CO. Until recently, like June of this year, AA had a lifetime elite program and CO basically had nothing to offer once you requalified for Plat. CO now has lifetime elite status and when I retire I hope to be both lifetime Plat on AA and lifetime Gold on CO. (I would have to live on an airplane the next 15 years in order to reach the 4 million BIS miles for lifetime plat on CO.)

During irregular ops, AA is better than CO, and Admirals Club agents seem to have more power to help elites than do President Club reps.

Obviously this issue is truly a YMMV issue. Would it make sense to do this if I did not fly so much in a year, of course not. It is better to have top level on one airline than middle level on two. However, on routes in which my experience tells me that I am likely to obtain an upgrade on either carrier, I can even price compare, because the fares are not always equal. For my circumstances, with lots of flying and an aversion to riding in steerage, having top status on two carriers works well.