That's why the argument can be made either way.
Modern targeting technology tremendously reduces workload on pilots in all mission types. That's why the Su-35 still carries a wide suite of A2G weapons, despite only having 1 seat. The J-20 being China's premiere fighter aircraft, I don't see why China would be saving money and going with 2 seats and less expensive electronics (which is why the Su-30SM is becoming relatively numerous in Russian service).
Who says it would have less expensive electronics? Quite the opposite. A prime example would be comparing the role of the Rafale in its single and two-seat version where you have the latest data fusion cockpit and it is a true, omni-role aircraft that's limitless in operational tasks. ATM, the J-20's function in strict stealth mode (if I'm not mistaken) is strictly A2A. Give it a mission that is relegated mostly to ground operation and a 2nd pilot is invaluable, especially when its role is now an offensive platform and not a defensive one. With all the new tech such as EW, AI, controlling drones from the aircraft and software technology as well as HMDS, no matter how easy it's designed to be for a single pilot, there will always be situations where there is just way too much sensory overload. There is no way around that when a pilot has to have situational awareness, fly the aircraft and combat an enemy, no matter how easier all these systems are made for the pilot, they will always be presented with situations where there is work overload, especially when the battle is taken deep into enemy territory. A single pilot limits the role of the aircraft no matter how advanced it is.
This is clearly evident in the F-22 and the F-35.
Also, larger aircraft (such as the J-20) that are relegated to very large combat radius and loiter time also pay a lesser penalty in fuel capacity for carrying a second seat than smaller aircraft. Given that premise, I would say that being a larger aircraft than the other current 5th gens in testing and production, it will also pay less of a price in RCS for a larger cockpit.
As to why not, it would seem to be a massive hassle to redesign the airframe just to accommodate a second seat, considering the J-20 is a stealth aircraft and that portion of the aircraft has to undergo more development to be made stealthy. Without radar absorbing coating, it may still be less stealthy than the single seater. China has some stealthy UCAVs in development...
A 2nd seat will definitely reduce RCS, can't argue that, but I think it would be minimal and the reward would be greater in return for mission-specific tasks. Look at the eventual FGFA for example. It's going to be a design that will sacrifice some stealth to have that capability of a 2nd operator because the particular airforce looking to acquire a 2-seat PAK-FA has certain needs and mission tasks that require a 2nd pilot and are willing to give up a bit of stealth for that. There's also the notion that when you lose a little stealth coming from your avionics and cockpit, you fly higher altitudes to help reduce that.
Also, as far as a massive hassle and cost, that might not necessarily be the case here since there are only 8 flying prototypes of the J-20 ATM. Granted they seem to be operational but it's still in developmental phase and we saw a major, redesign in structure and shape to the F-22 post testing and prior to entering production. If it's worth it to China since it's a huge country, why not? If it'll operate like the F-22 and F-35 which rely on huge outside support, then it probably won't need it.
Would be awesome, unbelievable ... 
