The correct answer is "it depends", mostly on both the type of codeshare agreement and the systems integration between the two airlines. In my experience with AA/LX and AA/BA codeshares, on both of these airlines the seat map for the codeshare is identical to the one for the non-codeshare flight with the exception that seats for the airlines' own elite customers appear as not-available on the AA seatmap. Also, AA/LX seats are pre-reservable only 191 days prior to the flight. Sorry, no experience on AA/JL flights.