(((((((((((((((hugs))))))))))))))))
I know what ever decision you make, it will be the right one.
I don't know whether this will help...my Schumi died naturally, but by the time he went, he was barely eating and very, very frail, constant mucky bum so constant cleaning (he was a gummy bunny, ate no hay). My Mum says he had lost his zest for life. She clung on because she didn't want to let him go. She now regrets not having him PTS, as cruel as it felt at the time, she thinks now it would have been more humane to not let him suffer.
I know Jane was really articulate about her similar situation with Tess and Stanley, maybe have a little look for her threads, they might give you some insight and help.
Remember, you have given them both the best life and care you could, and they love you for it.
xxx