It is ok to let one of them expire. Just don't try to apply using both of them

Also keep in mind that as a PNP applicant, you have signed a document stating it to be your intention to settle in the province that nominated you. You should preferably land in that province or have a flight taking you there shortly after your landing. If you land elsewhere and the immigration officer feels like you might not intend to settle in that province, they can notify your PNP to check up on you and they do and find that you have not settled in that province, they can report you for misrepresentation and you can lose your PR.