Assassins creed black flag is locked at 30 FPS on PC. Not sure if they are going to patch it or not. Your FPS readings are normal for that card, and you really don't need to SLI two 770's as one 770 is more than enough. Can't see how you could justify the extra expense for the gain you would get in FPS which in my opinion you wouldn't notice, since the game are running at max settings. The recommended PSU to sli two 770's is 850W with two 6 pin, and two 8 pin connectors. I mention this as you don't say which PSU you have.
OnePlus 6 review