No, these cannot be combined. If you have a promotion code it can be used to save even more money when you are purchasing new certificates with a credit card or PayPal.
When you have a Restaurant.com Card you must redeem it for a certificate of the same or lesser face value. For example, if you have a $50 Restaurant.com Card, you can redeem it for a $50 certificate, two $25 certificates, or five $10 certificates.