Seniority is always going to be a contentious issue when airlines merge.
What should happen here ?
Airline A has a successfull regional operation, new hires start as FO on B1900 and work their way up via Q400 to the most senior as A320 captains. There are no Boeing aircraft in the fleet.
Airline B operates a medium/long haul all Boeing fleet with 737, 757 and 777. Basically a sound company but struggling financially and in danger of going bust.
A buys B, Sort out the seniority.