@Mark3921
You can not change the cancellation policy for a confrmed reservation.
The guest could have taken another approach:
Make first reservation and pay. Then extend ("change" option) the reservation after 1 day (or later, to be discussed with you) for second payment. Then there is 1 cleaning fee (if any) charged, 1 reservation wiith one cancellation policy) and 1 review option at the end. If the second part of the reservation is not neccesary, then you can shorten the reservation ("change" option, and amend price in the change form if neccesary) and guest will be refunded unused nights + Service fees.