If I understand the question, I would have to say that getting my worst knee fixed first has made my former "good" knee feel a lot worse. Before, they both hurt and they both were unstable, but I favored the really, really bad one and consequentially that quad atrophied a bit while the 'good' quad at least got some exercise, being the first up the stairs etc. It hurt, but the pain was minor compared to the really bad one.
Now, almost nine weeks out of the first TKR, I am very much aware of the problems with the remaining knee. What used to be my straightest knee, the more stable of the two, is now seeming crooked and shaky. Somehere, on some thread, I posted a photo of the two legs about a week after the TKR, and the difference is very noticeable. The old one is crooked.
I think that is due to two things, in my case.
First, the new knee is so superior that it's amazing. It's straighter, it doesn't hurt as much and anyhow it's a different hurt. It' more like recovering from a serious knife wound than anything else. Not that bone on bone agony. It is stable, strong, and reliable. It doesn't fold on me. Makes my former 'best' knee look bad by comparison.
Second, I am suddenly more active than I have been in years. I go up and down our very steep, unpaved, loose rubble covered driveway five or six times a day. I go up and down stairs, do all kinds of things. Walk a lot more. Stand for longer periods of time, like hours compared to maybe ten minutes before TKR.
What this means, though, is that my bad knee is wearing out faster. It is making a step up a steep driveway for every step the new knee makes. I think it is going downhill faster. Thats just in my particular case, of course.
We joke about me no longer babying the knee trying to make it last longer, since I know I am replacing it in Sept. I think I need the TKR pain meds more for the original knee than I do for the new one.
My unqualified opinion is that if both knees are bad, you should get them both fixed while you still have insurance.
Who knows what your situation will be a year or two from now?
Right now you can do it, and you need it. No brainer. Far as I am concerned, anyhow.
Have you found anyone on here yet who wishes they had waited longer?