one way to merge seniority is by the percentage method.
Let the date of merger be the date of reckoning. (DOR).
Pilots joining after DOR slot in to the merged company.
The problem is always the pilots who joined before DOR.
One of the best way is to make a percentage of time served in the company (TS) to the total time of existence (TE) in the company.
Each pilot then gets a score in % as on the DOR = TS/TE x100.
Asuming the same tech criteria is used for both. Incase of different or multiple criteria, both companies have to be given the benefit by factoring the criteria for both with a weightage. That is If it is Release date, compute similarly a fraction for release date to the first relaease date for both employees like the Date of joining and multiply both or add both to get a number.
Sequence in numerical order descending, higher score being senior.
This score determines the sequence in the list as on DOR.
Any plan will have opposition if it does not suit someone. The aim is ti have least dissatisfied people.
Can also use a decision making matrix ... PM if required
Regards