(Here I'll use J-31/35 instead of FC-31.)
Just to respond to the bolded sentence:
Mainly because you would be copying the
, and pasting them right onto the J-31/35.
Due to WS-15 being tailor-made for the J-20 (which is a large-sized, heavy-weight fighter), it has to be large in order to power that beast of a jet. The J-31/35, in contrast, is a medium-sized, medium-weight fighter. Hence, there is only so much space onboard the J-31/35 to fit one WS-15 (seen below).
View attachment 116481
Speaking of having only one WS-15 versus having two WS-19s - Here are their respective estimated max thrusts, based on what we know so far:
WS-15: ~18-18.5 tons
WS-19: ~10-11 tons
We can see that having two WS-19s onboard actually provides much more powerful thrust to the J-31/35, compared to just having one WS-15 onboard.
Then, there are all sorts of computer systems onboard the J-31/35, which requires sufficient amount of power that can only be generated by the engines onboard as soon as they left their parking space. Having two WS-19s provides much more power to power those systems onboard the J-31/35 than having just one WS-15.
Furthermore, the demand for power will only increase further, as the J-31/35 is expected to receive further hardware and software upgrades in the coming years. Needless to say, those two WS-19s can definitely provide a larger amount of spare/reserve power for the J-31/35 to receive future, more power-hungry upgrades - While the opposite is true for having only one WS-15.
TL; DR: Having only WS-15 per J-31/35 is going to be handicapping - Perhaps even outright crippling - The J-31/35, thus causing the fighter jet to underperform and rendered incapable of fighting in the skies of today and tomorrow.