From what I saw at the demo at GenCon, and assuming that the rules did not change much, crew slots add to their guns fire(per the LTs manual), and it can be assumed that two crew slots do add together..put like this: once crew slot is well trained at quickly bringing the gun to bear, while the other is very good at ferrying the ammo and powder, both of which could end up messing with an gun crews ability to hit the target.
Other advantages seen at Gencon: Well..one advantage: Crew can repair slots.