When an EMV device has been installed a software plug-in handles credit card processing via hooks in POS. Split tender with multiple credit cards is only supported on a standard sales invoice, not for a sales order (workorder, layaway, backorder) which may include payments over time.
**This is a necessary security block to prevent looping out of the transactiion with a partial payment charged to customer's credit card in error.""
The workaround is to process the first credit card as a deposit/payment, then recall the sales order and process the second credit card to close out the order.
If you are trying to tender this type of payment on a workorder, use Shift-F9, then Over-ride deposit to make the payments. Example, recall workorder, press Shift-F9 to enter a $25 deposit for first credit card payment, tender that, then recall workorder and use Shift-F9 to increase desposit to $50 to make another $25 payment on a second credit card.
In case of a layaway, use Shift-F4 to tender first credit card payment, then recall layaway and use Shift-F4 to make second credit card payment unless layaway is being picked up in which case complete the sale tendering the remaining balance.
Note: The same issue applies if you use a Mercury/Vantiv Gift Card plus Credit Card since both payments pass through the EMV device. Make them separate payments by tendering twice as above.