Many stores like it when the employees multitask but working on the cash register is best to avoid it to the fullest because you must serve one customer, even on busy days. You can get confused about the change and give it to the wrong people. Then, complete each transaction correctly to start another one. Even when a single customer makes more than one transaction with you, complete the initial transaction and then go to the additional ones.
Source: Work - How to Give Back Correct Change