You would be leveling both at the same time if you were performing tasks that granted you that Job Points or Skill Points. As they would be mutually exclusive.
Also jobs would not need to go to 50. 10 could easily be the cap and once you got certain ones to 5 and then 10 you unlocked extra ones.
Also unlike the Job/Subjob system your Job and Class are on the same playing field, you don't get your power cut in half because one is to low. IE just because your Job level is too low your class isn't cut in half like it was in XI.