I used this rule before, and am trying to direct a friend to it, but can't recall what it's called.
The rule is: Flying from U.S. using one airline for domestic flight out to another airport in U.S. to catch an international flight on another, different airline - - I can check two bags on the domestic flight just as I would on the international flight at no extra charge by the domestic flight carrier.
Example: I flew from Dallas to Portland on American. Then on Delta from Portland to Tokyo. I had two bags to check, both of the weight that is maximum for an international flight. American had to check these bags for me and not charge me for 1. too much weight for a domestic nor 2. two bags checked when American allowed only one at no charge. American counter at Dallas gave me a hard time, the manager pretty much calling me stupid for thinking she had to do this. I called American customer service and after about an hour of this person and the other, it was confirmed that there is an agreement which requires the domestic flight carrier to check bags at the international carrier's levels / charges (none). I recall this being to promote tourist buying things while in the U.S. In the end, American at DFW did check my two heavy bags for no charge and I even got an email apologizing for the mess up and 2,000 miles added to my account. But it's been a few years.
Anyone able to identify specifically this rule? As I mentioned, I'm advising a friend; that friend is Vietnamese whose English isn't strong enough to work the phones like I did. Any help appreciated. ( I am myself still overseas and so difficult for me to work the U.S. airlines from here.)