You just look at all recent projects to see how far off a twin seat J20 is at present.
Look at the J10, J11B. How long after the first flight until the twin seater came out?
For the likes of the JF17 and F35 a twin seater has not been seen even though those aircraft are in production.
For a J20 twin seater to appear this early in the programme would completely break with all past precedents, and would be an extremely remote possibility in my book.
to expand on that further, a twin-seater will mean a required mission redesignation for the aircraft
something like the j-20 current prototype is obviously meant for single-seater
2nd, and if based on the current layout of trends of china's attitudes towards flankers and what china is investing in, which was studying the flanker airframe inside out and then turning it into j-11,j-15,j-16..etc for various roles, then it's kind of saying the next evolution stage of the chinese air force would consist of flankers as main workhorse. having said that, where would a twin seater j-20 come in for a place in this air force?
having said all that, i really doubt we will see much of a twin-seater j-20.