If you have split a customer using the "split" function and now wish to delete one or more of the customers, then go to Customers (top bar)>three dots(top right) >Merge Customers. 

• Scroll to the name of the customer that you wish to delete and tick the box next to it.
• Select the customer into which to merge the invoices.
• Select Merge Customers (button at top).
• Click on the bin symbol (delete) symbol next to the customer to be deleted.

See How can I split a customer, for example to track end customers under a reseller agreement?