I think the terms said that you can only get one type of mileage boost every year!

Annual Miles Boost℠ for Reserve
The close of the calendar year is December 31, without regard to the time of the year that the account is opened. This means that for the first year of Card Membership, the Card Member's Eligible Spending bonus period may be less than twelve months. If in any calendar year Eligible Spending on the Delta Reserve Card is $30,000 or more, the Basic Card Member will be awarded 15,000 bonus miles and 15,000 Medallion Qualification Miles ("MQMs" as defined in the Delta SkyMiles Membership Guide and Program Rules). If in that same calendar year Eligible Spending on the Delta Reserve Card is $60,000 or more, the Basic Card Member will be awarded an additional 15,000 bonus miles and 15,000 MQMs. The MQMs are not redeemable and only count towards Medallion status. MQMs can be kept by the Basic Card Member or gifted to someone else. MQM gift recipients must have a valid SkyMiles account. If within 90 days, a MQM recipient is not designated, the MQMs will be deposited in the Basic Card Member's SkyMiles account. Additional Card Member(s) are not eligible to receive miles through Miles Boost; however all Eligible Spending by the Additional Card Member(s) will count towards the Basic Card Member's spend level to reach the Miles Boost threshold. Card Members may be permitted to have more than one Options, classic, Gold, Platinum, or Delta Reserve SkyMiles Credit Card from American Express account; however, Card Members are only eligible to receive one annual bonus for each type (i.e., Options, classic, Gold, Platinum, or Reserve) of Delta SkyMiles Credit Card account from American Express.