For the US/HP merger both sides were represented by the AFA which uses strict DOH, putting most HP FA's lower on the combined list than most US FA's. What I don't know is whether there were any out of DOH order FA's on the original US side and if so how that was handled.
As has been mentioned, the new law has gone into effect since then and will govern any US/AA merger since at this point no employee classification on both sides has the same union representing both. That means arbitration if the two sides can't come to a consensual agreement.
Jim