I highly doubt that they start work on the 360 version after the japanese PS3 version is done, its likely that they will begin a little while after the PS3 demo is released, they probably just said that to stop PS3 owners saying that 360 affected the game (making it smaller/worse graphics, like they claimed about GTAIV).
I hope that they start it before, otherwise not only will 360 owners suffer with the quality of the port and the waiting time, but PS3 owners will suffer a long wait.