Thanks for bringing your My Best Buy questions to our forum! I'll be happy to answer them for you.
If you already received the $10 certificate for Assassin's Creed 4 and Fifa 14 for Xbox 360, you would not be eligible to receive them for pre-ordering and purchasing those titles for Xbox One, as the promotion is only valid on each title once, regardless of the platform.
If you were to purchase those two games this week in conjunction with at third Xbox One game you would be eligible for the $25 certificate for the Buy 3 Get $25 promotion.
Please let me know if you have any additional questions. Thanks for being a My Best Buy member.