I took your question to mean that each of you has your own Sail Card backed by your own credit card (you can set this up at embarkation).
If that is the case then you will get two bills under your door the last night. One for each of you and then you don't have to worry about settling up between you.
Most of the ships will let you access you statements from your cabin TV. We will either check our bill there or ask the customer service desk for a print out early the day before disembarking. That way if you have a dispute, yo can settle it early and avoid the huge line that begins to form in the afternoon of the last day.
We also make any payments to our account (from cash we didn't use or money won in the casino.) so that we don't have such a huge hit to our credit card...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013