hkfooey,
Each airline must have its own pilots. That's true.
But this conversation is about seniority lists. There is no requirement for an airline in Hong Kong to have a seniority list just as there is no requirement as to how many seniority lists any one airline may have. (Just look at what's going on with base opportunities and who is eligible for them)
Hypothetically, CX and KA will get to choose their pilots from a combined list. It would be a requirement for any pilot to join that company in exactly the same way CX based pilots resign from CPA when they take a base and sign on with the based entity. Yet they remain on the common seniority list. That may mean a new contract, it may not. Those above the Y intersection would have the choice to stay or move, but those below would be moved between companies (resigning and re-joining as required) as per the company requirements.
Which complies with the AOC just as the current basing arrangement does.