I don't think two seater J-15 necessarily has a separate production line. Volumes seen so far don't warrant it. Actually, volumes of J15 in total suggest not a production line but more of a station workshop kind of production method, where planes are built, several at a time within a hall, staying in one spot and various subystems and tools are brought to them if needed, depending on variant of the plane assembled.
If i had to guess, actually, i would wager J15s are made within the hall where J8 were previously made, up until 2011 or so.
Whereas most of the production concerning flankers is done in a separate halls, ones that were producing J11 throughout 2000s and early 2010s and are now producing J16.