http://www.dewxp.com/Details/FAQ#bonusAnchor
Confirmation that combos double it.
Edit: Towards the bottom it says:
The Dew/Doritos Combo Bonus
Available: October 15, 2012 – January 31, 2012
DewXP Accounts will be awarded a special bonus of (2) Double XP Matches if they input any combination of both Mtn Dew and Doritos codes during a single online code redemption submission.
The Dollar General Bonus
Available: November 17, 2012 – December 22, 2012
DewXP Accounts will be awarded a special bonus equal to (2) Double XP Matches if they input a code printed on the receipt from a single Dollar General transaction that has both a Mtn Dew 20-ounce package and a Doritos single serve package on it.
The Speedway Double XP Promotion
Available: November 1, 2012 – November 30, 2012
DewXP Accounts will be awarded a special bonus equal to (2) Double XP Matches if user inputs a code printed on a receipt from Speedway that has 3 of any combination of Mtn Dew 20-ounce packages and Doritos single serve packages from participating Speedway stores during a single in-store transaction.
The 7-Eleven App Bonus
Available: October 30, 2012 – December 31, 2012
DewXP Accounts will be awarded a special bonus equal to (2) Double XP Matches after engaging with the 7-Eleven app and reloading to full supplies after running through initial supplies. In order to reload to supplies to full, the consumer must be in a 7-Eleven and scan both a Mtn Dew and Doritos product. The consumer will then receive a code to bank at DewXP.com. After banking, the consumer will receive their bonus XP Matches.