Why do my two kittens want to pee on their bedding?

Hi I've two 5 mth kittens - brother & sister - both neutered who like to pee on their bedding-wash it they "mark" it again! Do you think they are trying to take owner ship of the bed from each other? They used to both sleep in it together but now only one fits! I'm reluctant to buy another bed & cause another fight. Usally the female gets the bed and the male sleeps on a platform at the scratchpost!! Also they like to play hiding in paper bags and eventually after a day or so pee on this too...I'm turning into Mrs Mop here...any suggestions?!