Both are ALPA, so the ALPA Merger/Fragmentation policy will govern as required by the Bond/McCaskill Act. The merger frag policy expressly forbids windfall gains in seniority. In the mid 90s, the Board of Directors deleted Date of Hire in favor of "career expectations."
Career expectations was inserted to prevent senior USAIR pilots from jumping into UAL wide body seats during the aborted USAIR/UAL merger of that decade.
Career expectations doesn't apply to two narrow body carriers since both pilot groups could only expect to retire in a narrow body seat.
I expect some sort of deal where all AS pilots hired prior to 2007 are listed followed by a ratio of the remaining pilots from each carrier with no bump/no flush provisions to protect the people who are already holding Captain seats at each carrier.