Sasidhar is correct.
FKs are there to enforce referential integrity rules. No referential integrity rules takes into consideration one child row with two alternative parents.
If this is a business spec it has to be manually enforced.
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Pablo (Paul) Berzukov
Author of Understanding Database Administration available at Amazon and other bookstores.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.