if that's the million dollar question....why not merge via aircraft type? i mean, if things are somewhat based on types of aircraft...well, then NWA has the 747's...DAL doesn't have the 747's but has 777's.....so NWA folks would stay on the 747's, DAL would stay on the 777's...but if each has 737's, then those particular seniority lists would be integrated 1:1....if anyone wanted to move from a DAL type plane to an NWA vice versa, then they'd have to fold to that particular seniority ratio to get onto that aircraft type.
There aren't going to be that many types of aircraft that are different, altho i could be wrong.. i haven't taken the time to look at what aircraft NWA has.
so, as the company gets rid of or brings in new aircraft type, the lists "merge"