I once posed this question and never got an answer - 3 pilots work for different airlines but you have no way to tell which airlines. One has the seniority to hold widebody Capt, the second to hold 737 Capt, and the third to hold 737 F/O. Which has the most longevity and which has the least. Which has the most seniority and which the least? After all, if longevity is the same as seniority the answers should be obvious.
Jim
Take the 737 Captain. It may be all the airline offers. It could a company 1/2 the size. All the pilots senior to him could have a relatively young average age, so while he advanced quickly by timing a cycle right, his position will be stagnant long term absent growth.
The 737 F/O at brand B could be in a position where 90% of the list ahead of him will retire in the next 15 years, in a company that cut all the unprofitable flying. Upgrade and then advancement in larger aircraft will take place rapidly.
Neither persons position should be changed by the merger, and neither persons advancement especially within flying that came with their company should be affected. You can never get there with relative position. DOH with fences and conditions can achieve it. Virtual mergers like Air France/KLM where employee groups remain separate is another. Some sort of Dynamic seniority list that reverts to DOH over time is another. Again, snapshots and to simplistic. Maybe all an 83 year old, non-pilot, with non binding guidelines can come up with.
ALPA has always opposed legislation that would create baseball style arbitration for contracts because the argument is a "neutral" could not understand the dynamics of pay and the effects of work rule changes on pilots and it would take control away.
The only reason it exists for merger policy is lack of ownership and politics. Enough ambiguity exists so that no stand need be taken and different positions can be argue to defend and serve the political need of the day.