Make sure that you are on the Billing section of your account.
Then click on the right corner the 'Claim Another Voucher' button.
After that, please input the second AppSumo code that you have, then click the 'Claim Another Voucher' button.
If the AppSumo code that you just added is valid, it will automatically be added on your AppSumo Voucher Code list.
Please note that you can't reuse a voucher code as it will not be recognized by the system. And a notification will alert you that the voucher has been used.
If you encounter any problem with the voucher, immediately reach out to customer support.