I believe this was contemplated but was not realised? imho it would be best to have all widebodies produced at TLS and all narrowbodies at XFW, without any useless flying around of green planes. BTW, because of that, the engines have to be fitted onto the plane at a much earlier stage which involves high costs due to the additional capital which has to be financed for a longer time.
Another strange 320-series deal between TLS and XFW: All A318s are built at XFW, but if the number of them goes past a certain predefined number, the equivalent number of 319s will be built at TLS. Rather confusing, huh?