Our most senior kitty, who turned 15 this year, has had epilepsy every since she was a kitten. (Long story, but basically caused by the rescue group insisting on spaying her WAY too young!)
She lived a fine, happy life with us and outside of accidents caused by break-through seizures when her medication was in need of adjustment, we had no problems with her at all.
Not until about a year ago, when it seemed she could not remember where the litter box was. I was constantly cleaning up accidents in the hallway, in the bedroom, in the living room, and not only pee.
She has been checked out by the vet, and outside of her underlying medical problem, she's fine--just getting a bit senile, and perhaps a bit of arthritis in her hips that makes climbing into the litter box a bit difficult.
So, about a year ago, we put her out to live in the shop, which has a cement floor and no carpet; much easier cleanup. (Mind you, she has her snuggly bed, food, water, a litter pan for those fluke times when she uses it, and the shop has no windows, but does have central heat and air conditioning, so she's perfectly comfortable.)
As she's aged, she's also gotten a bit cranky with the other cats, so really she seems to enjoy her peace and quiet out there. Sometimes I need to have the shop open for a few hours, and bring her back in the house so she won't get out into the yard, because she'd be sure to wander off and get lost or worse--she tends to just walk without paying attention to where she's going--and I think a lot of that is on account of the strong drugs that control her epilepsy. At any rate, the last time I brought her in, she had a raging fit the whole time--running up and down the hall "swearing" at everything and nothing. She calmed down at once when she went back out to the shop, so I no longer feel guilty that she lives out there alone nowadays. Naturally, I do go out an pay attention to her, and pet her, brush her, etc...
So--if you have a garage, or an area like that where you can make the kitties secure, warm and happy, it might be an option. I, personally, would look for any other choice but putting them down. You might also ask for a second veterinary opinion..they could have bladder issues...Best wishes.