If CO/UA believe the combined company will bring investor value or a big savings to the company they'll do it. UA would get a better presences in Latin America and GUM. CO would gain a NRT and FRA HUB and a great presences in LHR/Europe/Asia/SYD. Domestic both gain from each other ORD, SFO, LAX , IAD, EWR, IAH, DEN and CLE but CLE not that much. HQ would be IAH more pro-business. US/UA would be hard to seller because of their D.C. area HUBs. They'll would reduce over lapping route capacity thats were the savings would mainly come from. Biggest problem is the debt they would have.
TW was purchase to capture market and reducing over lapping capacity but it end up as reducing over lapping capacity. STL was not going to end up as a hub for AA, If did end as hub it would be like CLE.