I am going to try and write this in a way that makes sense... I work at WL so I don't want to lose you on technical terms...
Ok for the Land portion you can use 2 seperate CC's but you MUST make sure that you are NOT on express check out. Here is the thing, the hotel system can not take more than one card at a time, so you would only be leaving one CC at check in that would in turn "open" the account valid to charge. The Keys to the World cards will be numbered at the bottom, the last number signifies who's card it is (there will be a bunch of numbers and then 01, 02, 03 etc.) when you charge back to the hotel room the entire number will show on the bil. What you would need to do is get a copy of the bil the night before you leave (or morning of, just leave time) and go over the charges and either figure out for yourself who owes what or highlight (star, asterik, whatever) your mother-in-laws charges and go to the desk and have them figure out the difference. Then you can say I want X amount of money charged to this card and X amount of money charged to that card. This is why you can not be on express check out...it automatically charges the card on file.
As for the ship, the CC that is on the reservation will automatically upload to the ship the night before you sail, when you arrive onboard you can go to the Purser's desk and let them know that your mother-in-law would like to have her own card attached to her Key to the World and you would like you card attached to yours and your wifes. They can attach seperate cards to the keys onboard.
I wish there was a way to do that at the hotels, it would make life so much easier for you and for the CM's but they have not implemented a way yet for it to be done.
I hope I did not lose you on that explanation, if you have questions I will be more than happy to answer them.