Any twin seat J-20 would make me lose some (a lot of) faith in its computing and sensor fusion. When is a second pilot ever necessary for a stealth fighter tasked with exploiting its VLO mainly for air superiority? J-20 doesn't look like it's a good low altitude, low speed strike aircraft. Unless that second pilot is helping the control of autonomously directed drones, any requirement for one should be easily resolved with computing.
I did write an article about this last month actually.
In short, the answer is basically that no matter how much automation or computing power you have, the human is still the limiting factor in choosing what to focus on and what priorities and roles and decisions are made.
A single seat J-20 might be able to do the A2A role and the battle management role both well individually, but doing both at the same time might be more difficult. A twin seat J-20 otoh can do both the A2A role and the battle management role both at a hundred percent.