Currently, there is one limitation around party migration: if you migrate Alice from P1 to P2, it is expected that P2 does not already know about a contract of Alice (which can happen if Alice and Bob are stakeholders on a contract and Bob is hosted on P2).
So the answer to your question is: you can’t migrate both Alice and Bob from P1 to P2 (if they share contracts). That will change with 2.9