This article shows how to split a bill by the products each person wants to pay for.
You can also split a bill even between a number of people using the quick split button.
To split a tab by items, on the Summary view locate the tab and click the "EDIT" button.
Select the items that one of the customers wishes to pay for then press 'PAY FOR ITEMS'
Process the payment as usual.
You will then be taken back to the tab where you can continue processing payments for the remaining items.