One ship is at warp 8.60.
The other is at warp 8.63
Their speeds differ by more than lightspeed, yet they can still see each other on the viewscreen and target each other with phasers. How?
Once a ship goes to warp they should be untargettable, even the slightest variation in warp speed is millions of meters per second different, they shouldn't even be able to see each other much less target them with phasers. If they somehow can target each other, they should appear to be moving backward in time.
We can see the ships in the same shot, and never see any of that lightwave/going backwards stuff because it's impressionistic and hugely simplified. They can target each other because their computers think and calculate at FTL speeds.