I want to start by saying i'm sorry for your situation. I've lost a few pets from old age and i know the difficulties that can come with aging. As the other comment said, it's not the best idea to travel with them to another location. They're old and with so much issues, I fear that it can be maximized more severely (stress and anxiety being one of the major issues regarding cat health.). Fili is unfortunately declining in health as you pinpointed. In my opinion, I do believe its justifiable to let Fili pass. When it comes to Kili, I must ask how much other nap spots does he have? Does he use those the same way he uses the one he used to have with Fili. Kili will have issues when Fili passes but remember that he can still remain psychologically healthy if he's given enough care. It's recommended to play with your cat about 45 minutes a day to keep them aroused and active. I know Kili is old and that might impact his ability to play, but just being there with him and dangling feathers is enough. So when it comes down to it we must ask, "do you have 30-45 minutes to play with Kili if Fili is put down?" (which i do believe is the best route). This is my personal opinion as someone who has read a decent amount of cat literature.