PAC already has a huge order to fulfill before we can even think of exports. Remember, the capacity will be gradually increased from 10-12 planes a year to 20-25 a year so the second assembly line could be that.
I don't think PAC will be occupying itself with export at this point in time. However, as a plane is made of thousands of parts, we will have some capacity in excess which can be re-exported back to china for assembly there but I do not see an assembly line for export purposes at PAC for the near future. They first have to gradually build their capacity and absorb the technicalities required to achieve their stated share in the production and fulfill the PAF requirement.
That is their top priority.