In this case the customer has requested a "Credit Card Rollover" function. This means the supervisor would like their employees to be able to pay any balance that exceeds their allowance with their own credit card.
First, set up your allowance (previous step) and make the sort order 1. Next, you turn on the Credit Card ledger, then set the sort order for credit card two 2.
Make both sort orders 1 if you would like to give employees the option to use their allowance OR their credit card.