I personally would never consider a double knee replacement or double anything! If you have both done at once, you literally do not have a leg to stand on because it takes a while to get the quads muscles working again.
As it was, having one knee replaced, I still had problems eg getting in and out of a shower and in and out of bed going to the loo during the night. Think of the practicalities- will you be able to cope? I was on my own and for a while I had to have all drinks and food in the kitchen because I could not carry anything as I was using crutches.
It is also very painful post op so that is another consideration. Good luck.