There is no ~10 year technology gap like in-between F-22 and 35, however: FC-31(if it is the base aircraft for J-XY) and J-20 come from the very same technological level.And yes, what you described -- a "worse" J-20, with commonality with the navy, and a larger production run, more maintainable stealth, more advanced production method -- sounds like a hell of an aircraft to me, especially as such an aircraft would not need the additional development time needed for a clean sheet design.
So, if there is space for "larger production run, more maintainable stealth, more advanced production method", the same is true and applicable to a larger J-20 run(minus unwarranted commonality problems and huge development cost). If there is no such difference - well, then no difference indeed.
As of now, I can't see any advantages for FC-31 / J-20 mixture over a WS-10/15-centric fleet, unless something is seriously wrong with J-20 itself, Tejas-level wrong. There is no indication of this being the case.
If idea is to complement J-20 numbers with a more numerous stealth type - you may very well wait for WS-15 and make a single-engined counterpart to the J-20. Same dev costs, same or better commonality level(within 1 fleet, not two), more savings.
If you design a new aircraft from scratch, and don't want more stealth strike - basic WS-15 will be fine.And of course all of this talk of a single engine medium weight 5th gen aircraft operates on the understanding that it would need an uprated WS-15 variant to power it.
P. S. and as of now, from our current information, separate J-XY engine isn't ahead of WS-15 in readiness. Quite probably it's the other way around.