I've recently written a will, I get a death in service grant from my pension so have left a £10,000 lump sum from that for the boys' future care. I have stipulated that they must not be separated and that the absolute last resort would be for them to go in a rescue pen/cattery environment as they really don't do well in such an environment, who would choose a cat who hisses at them from a pen!
I've also written a lettter about their personalities, routine, their likes and dislikes so any new owner isn't left guessing.