Oh my....
it would be so easy and clean to implement with two simple checks while combining:
1.) If member has >75 nights in any of the two accounts before combining: don't grant (another) 75 night benefit (else: if combined nights >=75 then grant 75 benefit)
2.) If member has >50 nights in any of the two accounts before combining: don't grant (another) 50 night benefit (else: if combined nights >=50 then grant 50 benefit)
after that: grant benefits when crossing 50/75.
Done. Clean, easy, fair, easy to implement.