Hard decision about euthanasia?

I have 2 miniature schnauzers I am very concerned about, ages 15 years and 12 years old. The 15 year old (Dixie) is going deaf and blind, losing weight and seems to be having some dementia, as she has started eating poop and pee. She is almost incontinent, but with frequent trips outside we can keep the accidents in the house to a minimum. The 12 year old (Dandy) has lost bowel control and randomly drops poop anywhere, I don't think she is cognizant of it. She is also having difficulty using her back legs due to arthritis. They pretty much don't have quality of life, but have life. I am torn on how long I should keep them going or should I set them free. I guess I am asking your opinions, what would you do with the same situations? I know the decision will have to be made eventually, but not sure if I'm ready to make that decision.