If you want to eat in the castle it will cost you two credits/person. I'm one of those people who used to go every trip (four times last year). However with the new changes I won't be back. Too expensive, don't like Cinderella no longer be upstairs, don't like being forced to buy pictures I don't won't, miss Fairy Godmother downstairs, etc.
I don't believe you can split it and pay using 1 credit and then paying cash for half. You'll either have to pay all OOP or else use two credits/person.
As far as I know all other character meals are only one credit. FYI: they say Cinderella does not appear with the other Princesses at Akershus, but I ate both breakfast (Dec) and dinner (Jan) there and she was there both times. If you want Princesses I'd go for the Akershus breakfast (more Princesses and cheaper).