J-20 with two different powerpants?

On a related note, according to China Defense blog, eagle-eyed observers had noticed that the various photographs of the J-20 (with the serial number 2001) had been seen on different occasions two different types of exhaust nozzles (see photo), leading to conjecture the J-20 is powered by different powerplants, believed to be the Russian 117S and Chinese WS-10. This seeming anomaly might have been clarified today when two J-20 airframes reportedly put in an appearance together with both carrying the same serial number.

Photo: China Defense Forum via China Defense Blog

(Click on thumbnail for high res image)