You equip jobs to a separate equipment slot above your weapon (at least in 1.0; can't remember Alpha/Beta atm) . Like to be a Dragoon from my amazing Culinarian, I'd trade my frying pan for a spear and then equip Soul of the Dragoon to become a Dragoon.
The idea of 1 class = multiple jobs is that you'd be allowed to equip several job crystals while wearing a certain weapon. So, like, for the PLD/DRK branching off of Gladiator-- your Soul of the Paladin and Soul of the Dark Knight would say "Requires: GLA," so you'd equip a sword to switch to gladiator then equip your soul crystal. This isn't possible with 1.0's system though, as I don't think there'd be a way to differentiate between /job on and /job off. (Last point is largely irrelevant now tho)
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