Generally, taking jobs up at the same time is about as much effort as taking them each up to 50 one by one, with one exception: If jobs can share gear on the way up. Until you hit endgame, all of your Disciplines of Magic will share the same gear, both tanks will use the same gear, and both MNK and BRD share gear (DRG can share the MNK/BRD gear for the most part, though there's heavy gear out there which is PLD/WAR/DRG only, though the tanks typically get better making it effectively DRG armor).