If I have surgery on my left knee, and it is related to the disability from my right knee, can I get more disability?

If your other knee is causing the problems. i think it claimed as an secondary injury. But VA would be the best to talk to about this.

You can but the the va/military may revisit your claim and you could lose what you are currently getting for your knee due to them saying you are better due to the surgery. A dav rep told me this when I asked if I should add my neck surgeries and I am complete and total

You can get reevaluated but VA doesn't care what your Ortho thinks. They only care about what passes muster with their contracted doc you get sent to. Best bet is have a detailed report from your Ortho stipulating the cause and effect relationship between the left and right knee. Results will vary.