This article shows how to split a bill by the products each person wants to pay for.
On the Summary view, click the "EDIT" button on the tab that all of the items are on.
Select the items that one of the customers wishes to pay for.
Then press "MOVE ITEMS" and "NEW TAB"
The name for the new tap will be the same as the original tab with "[Split]" appended. If you don't need to change this, just press "OPEN TAB".
You will see that the items have been removed from the original tab and added to the new tab.
You can then take payment for each tab as normal.