When Worldpay returns a partial authorization - the terminal (not the computer screen - the credit card terminal) will show a warning that this is a partial approval - you have to press 'ok' on the terminal -  then the MicroBiz register will show the payment and allow you to select an additional payment or two so you can complete the transaction.

You can also split the payment across multiple cards before you run the first card - the payment amount generally defaults to the total amount but the clerk can edit this value.