I think it is individual, like many things. My doc wants us to concentrate on DHA for brain reasons. He wants' an omega 3 Index >10.5%. I got mine up to 17% last Nov. That was a bit over the top. My arms looked like I was on warfarin with a too high INR, especially as I bang them up rock climbing all the time. I backed it off a bit. Never had an impact on my afib.
I think EPA is more anti-inflammatory that DHA, for your purpose, but again, something to figure out. In your shoes, I'd likely start by increasing DHA gradually & see if I could find a spot that didn't aggravate afib, but helped the knees. Then I'd do the same with EPA. Hopefully finding a dosage that helps knees and doesn't make afib worse.