No, they won't split the order. Had 3 people from 3 different states in a villa, and the one who made the reservation ended up receiving all 3 bands.
You CAN use them to board Magical Express (in that case you would need them before you arrived at the resort) but you do not have to have the bands to do that. You can use the paperwork they send, or go to the DME desk and get a voucher for the bus.