That is assuming whatever components directly aft of the pilot seat that the WSO seat will eat into won't affect the jet's original capabilities like I mentioned.
I mean it's possible the space behind the pilot might just be free real estate reserved for an eventual WSO, but I find that unlikely. So like I said, whatever stuff they'd have to take out to accommodate jamming that WSO seat in, I hope it's not vital lol.
What will likely happen is that anything that cannot be fit by the addition of the second cockpit without causing internal structural changes will end up producing a slight hump to the aft of the cockpit, in the same way that most other twin seaters do it. J-10AS and twin seat F-16s for example have a spine ridge, while twin seat Flankers just have a larger hump immediately behind the cockpit.
That non-official CGI depiction of a twin seat J-20 is probably too simple because it doesn't accommodate for a rear hump -- whereas if we look at that brief CGI depiction from that AVIC video in June, showed a slightly more pronounced hump which would seem more likely to me (that said, I also wouldn't necessarily put money on the twin seat J-20 looking exactly like the depicted CGI.