Let me see if I can respond to some of the points you made, and perhaps provide a few more answers for you. Please forgive me if I get long winded here.
Yes at my airport UA DL do share the concourse, along with a few other carriers. The question you asked pertaining to responsibility for a breach, is one we have actually been discussing quite frequently as of late, as 2 of the hijackers from 9/11 flew out of my airport on the way to BOS. Honestly, I do not know how the cookie would crumble in a court if it were pressed. Although one could point to the custodian airline, and say "Hey they were responsible for hiring xyz firm!" in the end, each airline, as mandated by the FAA is responsible to ensure the security of its own operations. If say, my airline, had a history of logged complaints against said security firm, and the custodian airline, then perhaps a case could be made. I am sure this would be quite the legal mess.

Now, as for paying for increased security. Let me be clear, and try and explain something first. Airlines are provided with security requirements by the FAA. The airline goes out on the market and says... who can do this for $xxx.xx? A security firm says they can, and a contract is signed. That firm is being payed by said airline to ensure that the FAA directives are followed. The airlines job now, is to simply test security to ensure compliance. Now picture this purely hypothetical situation. XX airline goes to xyz sec company and says, your test object failure rates are way above where they are supposed to be, what's the problem? xyz sec firm says they can not adequately do the job that they said they could, for the amount they agreed to..ie lack of manpower. XX airline says, fine, although you did agree to this rate, we see the difficulty you are having, and you happen to be the only security company for 500mi, so we will pay you 25% over your contract to ensure we are flying safe airplanes. Mysteriously, the failure rates do not decrease, they are still dismal, yet it seems the local xyz company manager is driving a new car, and xyz security company is posting record profits. Do you see where I am going with this?
As for one airline wanting a different security company then the one the Custodian has picked, all that would have to be done is show non-compliance with FAA regulations, and have it documented with the FAA. They wouldn't be there long, OR the FAA would fine them into compliance. I don't see this being a big problem. I have never once even pre 9/11 had a security issue that was taken lightly by any airline. This really is one area where we do try to look out for each other, and speak with one voice if at all possible.
Just for future information, for you and any others that may be reading, all airlines are required by law to have their own security personell on site for every departure. These are GSC's, ground security coordinators. They are there to answer questions, clear problems at the checkpoints, and ensure guidelines are being followed. When you see an airline rep hovering around the checkpoint, taking notes, that's a GSC at work. If ever, any of you, ever have a problem, question, or issue with checkpoint security, simply ask them to call the airline you are flying on's GSC. They have a much more in depth understanding of the FAA regs, and a lot of training on dealing with security issues in general.
Your last question, payment for shared concourses. In a shared concourse a security firm will charge a flat rate for the entire concourse. That is then divided among the various airlines based upon their lift. All airlines track how many passengers they board from each concourse, and must report this to the DOT anyway. So, if WN flies 42% of the total passenger traffic from concourse x, they pay 42% of the security costs.
Once again, sorry to be so long winded, but I hope this answers a few of your questions.


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I do not speak for UA. The opinions expressed here are solely mine, and not UA's. Nothing said by me should be taken as an official statement from UA.