Assuming they keep the same overall airframe (which is the working assumption based on that AVIC frame of a twin seater J-20 from last year), then the primary advantage of having a second seat would be enhanced battle management and enhanced UAV control capabilities relative to a single seater.
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The second seat wouldn't really offer much more for the strike role, because a modern multirole aircraft can conduct quite complex strike missions even with one pilot (and especially so for a 5th gen aircraft) and also because the airframe shouldn't have any extra provisions for strike compared to the single seater either.
IMO the battle management mission really cannot be underestimated. Given the way air combat is going in terms of complexity, and given the inherent networking and sensor capabilities of a 5th generation aircraft, having an entire second pilot that is able to dedicate themselves and focus on the battlespace and assist friendly aircraft (and ships and ground forces/air defenses), while having virtually the same level of signature reduction and combat capabilities as a standard J-20, means you have a much more survivable battlespace/networking node that can operate closer to the "front line" and which is more dispersed/distributed in nature, than traditional AEW&C.
And of course, the benefits for controlling UAVs/UCAVs is obvious as well.
A single seater J-20 would likely have those same capabilities assuming their avionics suites are the same -- BUT, the single pilot would be hard pressed to simultaneously carry out its primary air combat mission and conduct the same extent of BM/UAV control missions simultaneously like a twin seater would, OR it would have to fly quite some distance from behind the "front line" to be able to focus on the BM/UAV control mission to the same effectiveness of the second seater in the J-20S.
However you cut it, assuming the avionics and sensor suites in a J-20 single seater and a J-20S twin seater are the same, the "capacity" to perform the BM and UAV control mission under
equal combat conditions will be substantially greater in the J-20S simply by virtue of having a second additional human being in the aircraft.