You can scan barcodes with your phone's camera too. But if you use a bluetooth scanner on your iPhone, it will only scan the items into your phone. 

If you use Square for Retail though, you can save carts! So everything you scan on your phone and save to a customer's cart, would be accessible on any other device that is logged into the same location. You would scan items into your phone for checkout, save the cart with a note to know who's cart it is, and then open the cart on your terminal to finish checkout!