I would suggest that they can be pre loaded. You can expend bonus actions to reload one at a time.
Round one thief rogue: main action attack, bonus action attack, then bonus action reload.
Round two: bonus reload 2nd xbow, main attack, bonus attack.
Round three: bonus reload, bonus reload, Main attack.
con’t…
Most rounds would get a main and a single bonus attack.
That’s the best way for this to work. It makes the one handed xbow more of a thief weapon or a fighter with extra action. The one handed xbow is a great bonus weapon if you have extra bonus actions.
