Multiple discount codes per order
It is not possible to enter/use more than one discount code per order.
If you want to give a customer more than one type of discount, you can handle the situation "manually".
This can be done by creating a unique discount code for each customer, where the amount is locked to the discount that the customer should receive on an agreed order.
It is also possible to change an order after it has been placed. The customer must place the order and pay the full amount (the money is reserved), after which you can change the amount on the individual product lines before the order is dispatched and the money is withdrawn.