Allergies can and do creep up at any age. I know what I'd do if I developed an allergy, I'd still keep the animals that I made a life long commitment too when I bought them...

With respect, you should be careful in how you rehome these two. Don't want to upset any one but some times not so very nice people will trawl the internet looking for cats free to good home to use as bait in dog fighting. Also make sure they go some where that isn't going to pass them on again. Free to good home attracts impulse buyers. These are senior cats and as I am sure you appreciate need a secure and happy home.

I'd give your vets a ring they may have numbers for local rescues and be able to help. Also try the cats protection league. If your still not having any luck I have fostered cats before and may be able to offer some help.