I recently wrote about being sure to use up all of your on board credits.
In the past, the on board accounts of couples in the same cabin, on the same credit card, had their accounts added together. So if one had credits on their account, the credits would be applied against the others' charges. Recent Princess passengers have reported checking their statements mid-cruise and finding just that. Until the end (see above referenced post).
According to the thread I am reading, one of the forum member's TA called Princess. The explanation is that the OBCs (on board credits) have always been assigned to one person and are non-transferable. Princess is now going to enforce this policy.
So it is important to know which passenger has the credits and how much. If it looks like there will be a credit at the end of the cruise, it is possible to transfer charges to the account with the credits but the credits, being non-transferable, cannot be moved to any other account.
If, at the end of the cruise, your charges have been charged to your credit card, leaving you with unused credits on another account, call Princess customer service post-cruise to request the unused credit.